So my family (mom and two brothers, all adults) adopted a pitbull puppy (19 months old) a little over a month ago. Well he warmed up to us super quickly and was super excited to come home with us and even did spectacularly well during the hour-long car ride back home. He has taking a liking to his new home and seems pretty comfortable now but anytime my boyfriend comes over he gets extremely defensive. He doesn't outright bark or leap at my boyfriend until we start cuddling or getting close though, before that he just eyes him carefully and follows him around the house. The same thing happens when my mom brings over her man-friend as well. At first I thought he was just wary of my boyfriend because he isn't part of the family and wasn't there when we got him, but my boyfriend practically lives here. He is here almost every day and spends 6/7 nights a week at my house and I would have thought Max (our pit's name) would have gotten used to him by now but he hasn't. How do I curb this behavior before he actually hurts my boyfriend or any guy my mom brings over?

EDIT: guys, the "get a new boyfriend" comments aren't funny anymore.