A good gift for a boyfriend's parents for Christmas would be a nice bottle of wine, for example, a well-reviewed red wine. But it doesn't have to be expensive. Any gift for the parents can be just a little something, as long as you're not empty-handed.
A box of chocolates is usually always well received. However, if you want to be more original, then try getting them a gourmet food item, for example, a tin of popcorn that comes in uncommon flavors. Or choose another food item or dessert that you think will wow them.
Don't forget about the pets either. Buying something for the family pet such as a juicy bone for the dog is another solid idea.