My husband's son

My husband and I have been married almost 7 years. He has two grown children - a daughter almost 31 and a son who just turned 30. If you do the math, the kids were adults when I came into the picture and my husband had been divorced for 8 years when we met. The kids have a mother and I never tried to become their mother. The daughter and I get along great - no problems there. But the son - oy! He and my husband, at least since I have known them, have had problems. Yes, it takes two to tango, but the kid has some serious problems. Not sure what the clinical term is but he things everything he does is right and everyone else does things wrong. I have been very silent about the way I feel about the kid but inside I want to "slap" him into reality. The latest thing is after almost 4 months of no contact from him even when we tried, he called today and was immediately forgiven for all the crap he has put his dad and I through. I have tried to do everything I can so that they can have a relationship but I was accused of trying to get in the way. If there was enough time I could tell you what I have done for my husband, his kid, and his 96 year old mother. I am at wits end - I don't know how to handle this! HELP!