All cats may hide under beds for a week. I don't know how much one can do about that. When I moved back home, one of my cats had already lived here for 11 yrs, and she was like "Oh good, we're back here. When's dinner?" the other had only ever lived in my apartment and while he visited my parents with me every weekend, when we MOVED here, he freaked out!! How he knew the difference, I don't know. He hid under my bed for a week. There were two other cats in my parents house who he didn't know very well, though being very young he still was cool with other animals.
Maybe rotate bedding back and forth between the cats so they get accustomed to the smells of each other. You may let them encounter each other on their own (supervised) terms once they're comfortable with the house. As in, don't just grab two cats and stick them in each other's faces. When they are together, lavish them all in treats and love, and hope for the best! Your cats may get used to your bf's cat before your bf's cat is used to them, considering yours are younger and have not been only cats! Good luck!