Every time I come over my boyfriend's place and my boyfriend and I hang out the dog suddenly becomes overly excited. He doesn't seem aggressive towards me because we get along just fine usually. But when, for example, my boyfriend and I sit down and watch a movie on the couch he becomes mischievous and excited. When I give him attention he gets too rough and when I don't mind him, he barks and lunges at me. It seems to me that he just wants to play with me but he gets too rough (he's heavy and he play bites) and it starts to scare me a bit as he is a big dog (american bully). I try to set boundaries but it doesn't seem to work. My boyfriend observed this behavior only when I am around. How do I solve this problem?
The behavior of the dog is extremely impolite and disrespectful, without trying to sound alarming, this is neither funny nor acceptable, even if it is playful and not aggressive, because it will become aggressive and dangerous if unchallenged. The behavior you describe indicates that he sees you as a playmate and not a pack member and definitely not as pack leader, which should be the case.
To be honest I normally tend to side with the guys, but in your case it's quite clear that it's mainly your boyfriends fault. He apparently didn't introduce you correctly to his dog as a second pack leader or at least pack member and doesn't seem to do much to change the current situation by setting the boundaries and limitations for his dog. But he's key in helping you to claim your role in the pack, because it would be quite difficult for you to change the dogs behavior on your own. I'm far from insinuating that your boyfriend doesn't respect you, but that's exactly what he communicates to his dog through his inaction/unwillingness to correct the disrespectful behavior. And why should the dog respect you if his pack leader apparently doesn't do either.
However, your boyfriend won't be able to establish your roll in the pack if you're not willing to fill that position. Thus it has to be a team effort. He has to put his dog in his place and support you in setting the boundaries and limitations for the dog.
First of all, prevent jumping and no playing in the house. Don't interact with the dog if he's overly exited, if the dog doesn't leave you alone when you ignore him, it's the perfect moment for your boyfriend to step in. If you play with the dog (outside), it's you who decides when to play, what to play and for how long.
Secondly start showing the dog that he's on the lower end of the hierarchy. He's not allowed to sit on the couch with you or sleep in your bed. This must be a strikt rule until his behavior changes significantly. I know it's hard, but it's key in your situation. He eats after you've eaten. You enter or leave the house first, you walk in front... all the Cesar Milan stuff TBH. This might confuse your dog in the beginning and he'll have to cope with the new situation where he's stripped of all of his privileges, however his behavior should change quick if not instantly, if done right. When everything is going in your favor, you can consider to let him regain some of his privileges, for example he's allowed back on the couch, but only with your explicit invitation...