That's not a good situation. I've dealt with my fair share of them and have learned the best thing you can do is to keep them out of your life as much as possible.

Does he live with you? If he's 18 is he moving out soon?

Be very careful if you try to talk about this with his dad--family members can sort of block stuff out of there head when it comes to their loved ones.

Don't try to "protect" his family beyond that--it'll just cause complications, and may turn him against you--and sociopaths are dangerous in complicated situations. They will use everyone's emotions to their advantage.

What is some of the behavior you're witnessing?