Aggressive Rat Terrier

I picked up my dog from the humane society about 3 years ago. At the time I lived alone and he made a great companion.

Now I live with two roomates that he loves and is a great dog until someone sits down on the sofa with me. He turns very aggressive towards me. He bares his teeth, growls, barks, and sometimes bites me. What do I do?

I don't want to give him up and I don't want to move out, but something has to happen otherwise I'm gonna have to be forced into putting him down.