Practice collar grabs with treats - reach towards his collar, give a treat, grab his collar give a treat, rub his neck gently, give a treat. He should bounce back quickly and when he is working well for you, have your boyfriend do the same.
From now on, the roommate's boyfriend has ZERO contact with Atti. Though I'm sure you were already going to do that.
I understand this particular frustration. I moved in with some roommates who all seemed decently knowledgeable about dogs - not experts or anything, but seemed to have common sense. Well, the first night I brought Kaiju home I put him in his X-pen and came back into the room to find them whacking him i the face with their hands, encouraging him to chew hands. I yelled at them about it, they apologized. The next day, I came back from running an errand to find them handing him a Nylabone, them ripping out of his mouth and laughing when he screamed for it. I stopped any interaction between them at that point and crated him in my room when I was gone. Found out a couple days later they were going INTO MY ROOM and taking him out of the crate to do the same things I was yelling at them about. Ended up having to padlock my room when I was not there.
At least the boyfriend does not live there it seems like? If his general attitude towards your puppy continues in this direction, I might have a serious talk to your roommate about the boyfriend simply NOT being allowed over at the apartment.