Msinger, if you are not already seeing a psychologist, I suggest you do so -- for a visit or two by yourself -- to obtain a candid professional opinion on what it is you and your son are dealing with. Again, I don't know what is wrong with your BF. But, if I were you, the very first thing I would start reading about -- while waiting for an appointment -- would be the symptoms of BPD (Borderline Personality Disorder).
Granted, the moodiness and angry outbursts you describe could be caused by a hormone change (as men experience at puberty or in middle age) or by drug abuse. And very very rarely, they could be caused by a brain tumor or a recent brain injury. Yet, because you mention nothing about such events occurring, the two most likely causes of such abusive behavior are bipolar disorder and BPD.
I am not a psychologist. I can only share with you my experiences of living with a BPDer exW for 15 years and taking care of a bipolar foster son for longer than that. Moreover, I took both of them to a long series of psychologists for 15 years. Based on those experiences, I have observed twelve clear differences between the two disorders.
One difference is that the mood swings are on two separate spectra having very different polar extremes. Whereas a bipolar sufferer swings between
mania and
depression, a BPDer flips back and forth between
loving you and
hating you. Significantly, you mention nothing about mania. Instead, you describe him as flipping back and forth from loving you and devaluing you.
A second difference is seen in the frequency of mood changes. Bipolar mood swings are very slow because they are caused by gradual changes in body chemistry. They are considered rapid if as many as four occur in a year. In contrast, four BPD mood changes can easily occur in four days. The latter therefore seems consistent with your description of numerous temper tantrums.
A third difference is seen in duration. Whereas bipolar moods typically last a week or two, BPD rages typically last only a few hours (and rarely as long as 36 hours). Again, these short-duration rages seem consistent with with the tantrums you describe.
A fourth difference is seen in the speed with which the mood change develops. Whereas a bipolar change typically will build slowly over two weeks, a BPD change typically occurs in less than a minute -- often in only 10 seconds -- because it is event-triggered by some innocent comment or action. As you say, his outbursts appear to be "event-triggered outbursts." This would explain, then, why you say you "can't really discern a cycle of any sort." That is, there simply is no cycle to observe.
A fifth difference is that, whereas bipolar can be treated very successfully in at least 80% of victims by swallowing a pill, BPD cannot be managed by medication because it arises from childhood damage to the emotional core -- not from a change in body chemistry. Indeed, 70% of BPDers (i.e., those having strong traits) report having been abused or abandoned during early childhood. This seems consistent with what you say about your BF's childhood. His father abandoned him at birth and he was subsequently abused.
A sixth difference is that, whereas bipolar disorder can cause people to be irritable and obnoxious during the manic phase, it does not rise to the level of meanness and vindictiveness you see when a BPDer is splitting you black. That difference is HUGE: while a manic person may regard you as an irritation, a BPDer can perceive you as Hitler and will treat you accordingly. This seems consistent with your description of very hateful, spiteful behavior.
A seventh difference is that, whereas a bipolar sufferer is not usually angry, a BPDer is filled with anger that has been carried inside since early childhood. You only have to say or do some minor thing to trigger a sudden release of that anger -- which seems consistent with your description.
An eight difference is that a bipolar sufferer typically is capable of tolerating intimacy when he is not experiencing strong mania or depression. In contrast, BPDers have such a weak and unstable self image that (except for the brief infatuation period) they cannot tolerate intimacy for long before feeling engulfed and suffocated by your personality.
BPDers therefore will create arguments over nothing as a way to push you away and give them breathing room. Hence, it is not surprising that they tend to create the very WORST arguments immediately following the very BEST of times, i.e., right after an intimate evening or a great weekend spent together. You don't comment on this, however.
A ninth difference is that the thinking and behavior of a BPDer includes more mental departures from reality (called "dissociation") wherein "feelings create facts." That is, BPDers typically do not intellectually challenge their intense feelings. Instead, they accept them as accurately reflecting your intentions and motivations. In contrast, bipolar disorder tends to be more neurotic in that the mood swings tend to be based more on extreme exaggerations of fact, not the creation of "fact" out of thin air based solely on feelings. You don't comment on this.
A tenth difference is that a bipolar sufferer -- whether depressed or manic -- usually is able to trust you if he or she knows you well. Untreated BPDers, however, are unable to trust for an extended period. Before they can trust others, they must first learn how to trust and love themselves. Sadly, this lack of trust means there is no foundation on which to build a relationship. Moreover -- and I learned this the hard way -- when people cannot trust you, you can never trust them because they can turn on you at any time -- and almost certainly will. Significantly, you say that "he does seem to mistrust me." You say he has gone through your phone, questions your whereabouts, and has been jealous of several of your male friends.
An eleventh difference is that, whereas BPDers are always convinced they are "The Victim," bipolar sufferers usually have a much stronger self image. BPDers therefore have a strong need to validate that false self image by blaming every misfortune on the spouse. This behavior seems consistent with your complaint that he seems to blame you for everything. Specifically, you say "It ranges from criticisms of how I parent our child, how I do housework, how many hours I work, etc. to full on screaming or yelling and calling me names or stupid, sometimes in front of our son or in public. I know that this has been a pattern for him, in past relationships." You also mention that he still seems to feel that his exGFs victimized him too (i.e., "he speaks horribly" about them).
Finally, a twelfth difference is that, although bipolar sufferers are emotionally unstable, they generally are not immature or childlike. BPDers, in contrast, are so immature that their emotional development typically is frozen at about age four. This is why they have a very fragile self image and have difficulty controlling their emotions. Although I started out by asking whether his behavior seems unstable, you did not address that directly -- perhaps because you don't know what I mean by "unstable."
I therefore suggest you read my description of instability and other classical BPD traits in my post in Maybe's thread at
http://talkaboutmarriage.com/general-relationship-discussion/33734-my-list-hell.html#post473522. If that description rings a lot of bells, Msinger, I would be glad to discuss it with you and point you to good online resources and books.