Unless that's all he gets, that's not too weird. I had a boyfriend who always asked me for new boxers for christmas because he just didn't like shopping for them. I have no idea why. So I'd always buy them and he was always excited about getting some. Also, maybe his mom just wants to buy him something she knows he needs and won't buy for himself.
I don't know about there, but where I'm from its a specific tradition. It goes like this: When a family member asks you "what do you want for Christmas?" and you say "I don't care" or "nothing" then they get you socks/underwear. Basically it means if you want something for Christmas... anything... but you refuse to say it because you don't want to inconvenience the giver or you figure you can buy it yourself... then you deserve socks/underwear for Christmas. Here people usually get what they ask for even if it sometimes is outrageous like a car or a stereo system. The whole family may pitch in for that one present. Kind of strange I know but its just one of those spirit of Christmas traditions.
I doubt its the same deal with him and his mom. I just explained mine.