I actually think you are very right to feel the way you do. With your H using porn since the age of 12 he will most certainly be addicted. Not only that, but it will also more than likely be the cause of him not being able to have a normal sex life with you, his wife. This latter is for example a massive problem with men in their twenties. They simply cannot get an erection with a “real” woman. As a man of my age this simply did not exist, it was not a problem so I enjoyed probably the healthiest sex life possible with my wife.
Porn use by those as young as twelve is also a very big problem and a problem that’s getting worse. To the extent that there are Governments considering putting the onus on the porn providers and enablers to prevent children using it as well as the parents. That is, Governments are considering legislating that ISPs block porn unless specifically asked for by an adult. There are cases now where gangs of boys as young as 12 and 13 raping little girls. This is a massive problem and I think you are doing your duty as a mother to shield your children from it all and as such you should be applauded, I certainly do. I feel for those children who don't have mothers such as you to protect them.
I also feel for you in that your husband isn’t shoulder to shoulder with you on this. It must be similar to having an alcoholic as a husband and worrying that your children will start drinking and become one as well.
On top of all that you have the betrayal of your husband's lies and deceits to deal with as well which is a whole big thing in and of itself. Deceit about the use of porn is one thing, lying about it is a massive betrayal.
But what can you do? Boundaries are perhaps the way ahead, read up on boundaries in marriage. But goodness knows what you do if you assert some, you probably already have, and he just ignores and breaks them.
He wont change unless he sees he has a problem. So maybe educate him. Find out all you can about how porn affects young children and sex in marriage, print it off and give it to him to read. Find out about support groups on the subject, it’s such a big problem and getting bigger I’m sure support groups are springing up to provide help for mothers and wives.
Other drivers for change are pain, fear of loss and fear of failure. If for example you demonstrate to him what it means to lose his family that may cause him pain enough to change his ways. Maybe tell him you want a six month separation to sort things out and you’ll review how you go forward in six months time. Let him live his life without constant access to you and the children and see if that causes him enough pain and fear of loss to initiate change.