It all depends on how long he has been your boyfriend, how serious, if at all, the relationship is, and how well you know his parents.
If the boyfriend is a relatively new person in your life, and this is the first time, or one of the first times, you will be joining his family for an event, then a small "gesture" gift is the best choice, like a bottle of good wine, or if you are on a budget, and know a little bit about wine, an interesting one that they might not know about, like one of the delicious but inexpensive Bulgarian reds!
If the event will be at their home, and they have a Christmas tree, a little looking around should net you a unique handmade ornament from some unlikely corner of the planet, or maybe a pretty basket of fancy holiday baked goods from a foo foo bakery, or if you are a cook, or your own heritage has an interesting culinary tradition (and I can't think of one that doesn't) then a pretty dish of jaleebis or tamales or hey - a mixture of both, why not?
In any event, you should consult with your boyfriend first, this is not the best time to surprise him, for example, you would not want to give a bottle of wine or anything alcoholic if either parent is a recovering alcoholic or simply does not drink alcohol for whatever reason, and the baked goods should be savory ones if either parent is a diabetic or on a no-sugar diet, you get the idea.
If it is a serious relationship and you already know the parents, then presumably you already have an idea of their interests, likes and dislikes, and should be able to choose a gift for them much as you would for anyone else on your gift list.
However, you would still want to avoid gifts that are too personal or too expensive, and stick with that even after you become engaged.
Once you are married, his parents are your parents, and you may give them anything that you think they would like, just as you would your parents.