Rescue Boston Terrier does not like room mate's boyfriend

I rescued a Boston Terrier a little over two weeks ago. All I know about his past is that he nipped at the baby where he lived. It is clear that his last house did not train him very well. He has lots of love to give, but still needs basic obedience training, housebreaking, and has some aggression issues. The dog has growled at new visitors, but quickly warms up to people after a few minutes. We start obedience classes next week which I am certain will be a big help. (they can not start soon enough in my opinion)
The current problem is my dog does not like my room mate's boyfriend. The problem arouse a week ago when my Boston greeted him at they door and my room mate's bf accidentally slammed the dog's head in the door(not too hard though). Last night the bf come over and the Boston barked, snapped, and even bit the bf (he bit and held on and would not let go. He did not break the skin on him, but when I pried his teeth off I got a nasty bite on my finger). I told the bf to not approach the clearly aggressive dog, but he did anyways. I have been keeping the dog on his leash in the house to stop his indoor peeing and to keep him from chasing the cats where I live. I probably should have just said a loud no and gave a sharp yank when dog starting growling and barking, but I was surprised by his extreme aggression. After the incident I calmed the dog down and put him in his crate for the night. I was not sure of how I should have reacted and hope to be able to do the right thing. I am certain classes will help too, but in the meantime, any advice would be appreciated so I can try to nip this behavior in the bud.