I'm concerned that when I die, it will be exposed. Secrets are mostly harmful. Should I confront his mother?
His well being is paramount to mine. His mother may have already told him.....
Well, It's not easy to walk up to your 22 year old son and say "I need to swab your mouth....".
BTW, He already knows his mother is a cheater. He has witnessed it.
May I offer a couple of suggestions. First, unless you absolutely know beyond a shadow of doubt, I would not tell him. Even then, I still probably would not tell him. If it comes out after you die, he may silently say "thank God, Dad never knew as it would have killed him."
Now if you want to continue to be a "great father," Go to either Ancestry.com or 23&Me.com and take the DHA test for yourself. Don't ask him to take one. The advantage of the 23&Me DNA test is it is more medical condition related than the Ancestry. The Ancestry is more family connection and family tree building related.
Then after you get the DNA results back start doing some Genealogical research on YOUR family tree. The gift of a family tree and photos of relatives is a wonderful present for an adult child and their future family. My mother was adopted. After her death, I got a court order to unseal her adoption and all related court records. I got some interesting and horrifying news.
My mother's adopted parents were the only parents she knew and she never wanted to know anything about her biological parents. In my family tree I trace my mom's adopted parents families way back in time as they are the source of much of my cultural heritage and my maternal grandmother lived with us while I was growing up.
I also have on the family tree my mom's biological parents. I have have the prison mugshot of the old man who raped a 13 year old girl that was the creation of my mother. A court order can get you almost anything, including old court records, prison records, and adoption proceeding minutes. Your son will hopefully not be as shocked as I was about his biological past.
I would wager that since you say he knows his mom was a "cheater" he may have at some point questions. Then again, he may already know, but doesn't want to hurt you. When those questions naturally arise, if you have a DNA test from one of the major labs, he will be able to get answers easily about his biological relationship to you. Don't force this on him, let him want to find out and provide him with the information when he wants to know, even if it is after your death.
If you decide on this course of action, then do it as it will be able to answer lots of questions your son my have after your death. If you do this, then you might also at some point in the future ask your son's mother to also do one of the major DNA tests, not tell her son anything, but also just leave it as a legacy for him to discover at some point in the future.
My wife took the 23&Me DHA test and has found a huge number of biological cousins all across the country. She was into Genealogy well before I was. One of the most surprising results was a DHA close relative who contacted my wife and said that she was the product of her mother's affair during the time of the Korean war and asked if she might know who her biological father was. My wife put "two and two" together and contacted a cousin, who turned out to be a half-sister to the person asking. The two half-sisters have met and communicated now for a couple years. With modern DNA testing being more common, folks are finding out that there were a lot of illegitemate children out there.
The point is it may not be as horrible as you fear.
Good luck.