This is often a problem for blended families and it often does drive a wedge between the partners. He has to see this or this will become a huge problem. You tried therapy focused on his issues. These suggest that he hasn't had positive family experiences or good role models in this regard. This limits his ability to function in a healthy family environment. I am wondering if these limitations impact his need to feel threatened by your children. Is he having a sort of territorial way of looking at your children. He may be jealous of your children but he may also feel threatened. Even though he is the husband he is seeing the love you have for your children as robbing him in a way. He sees them as taking love away from him.
Marriage counseling is more appropriate in this regard His feelings of insecurity are pronounced when the children are involved so this has to be examined by a professional. This isn't about his anger alone but how to have a functional family life with your children and him. You can't handle this on your own. Find someone in your area willing to see both of you.
If this has been helpful press accept