Ask your boyfriend what he thinks? If it were my son, I wouldn't expect anything, it would just be nice to meet you. But if you wanted to take something, maybe a box of chocolates? A scented candle? Whatever you decide, best of luck. x
I always used to take their favourite bottle of wine if I was going to their house. Find out what it is, or at least if they prefer red or white. No gift bag, just casual, hi nice to meet you and hand it to them. Simple gesture, considerate without trying too hard.