Hi, I'd like to help you with your question.
It sounds like your son is using you to vent his frustration with his childhood. For some reason, he does not feel it is acceptable to put blame where it is due (on your father in law and his family). It may be that he senses that you will accept his anger so he puts it on you instead of taking the chance that others in the family will be angry at him and defend themselves.
Your son needs to start accepting responsibility for his own actions and the bad things that happen to him. You are not responsible for his anger or what happens to him. He is an adult and needs to deal with his feelings about his childhood and what occurs in his life as a result.
There are a few things you can do to respond to your son. You can suggest counseling to him. If he feels so strongly that his life was ruined, then counseling will help him feel better and be able to take charge of his life. You can also offer to participate in the counseling if you feel it would help. The other option is to start shutting down when he tries to blame you. Think of a neutral response you can have to anything he blames you for. For example, if he says you caused him to be overweight, you can respond by saying "I'm sorry you feel that way. It's good you are an adult now and have more control". Then walk away. Do the same if he blames you for his diabetes or other things.
If he tries to continue the argument, say the same thing. The more you repeat it, the less he will try to agitate you.
I hope this has helped you,Kate