Hi. While I don't have kids of my own, I work with families that have similar problems each day. I work with a lot of kids going through the justice system and I can see the impact that this has on their families. One of the most common things I see though is children who are traumatised and how this has affected them as they grow up. While I don't know anything about the situation your partner's son has been through, I can only imagine the pressure that his behaviour has put on his dad and yourself. I dont know if this is helpful at all but trying to understand why he behaves the way he does can be a start. When the families I work with finally understood exactly why their child/children were behaving in that way and how best to deal with it, it went a long way to repairing the strain on the relationship. The most important part of ensuring that his behaviour doesn't put unnecessary strain on your relationship, taking time out for the two of you and making sure you have someone to talk to about how you feel.
If you want to contact me via private message, i'd be happy to pass on some information for you about ways you can deal with his behaviour.
Hope you get some relief soon!
Diagnosed with SLE and Antiphospholipid Syndrom - June 2010