As a 20 year old, this topic has given me quite a few laughs!
Ellenoregon's son is not going to be the first or the last young American male to set foot in Spain wearing baggy pants. The reaction he gets is going to depend entirely on what part of Spain he is in, and who sees him. If he wants to ensure that he will not get picked on by any Spaniard, he should find a good pair of tight-fitting jeans, preferably boot-cut and with the "whisker" wash, a good pair of comfortable shoes that aren't tennis shoes, and a nice, button-up shirt with a collar (not too dressy, not too casual) or a tight t-shirt with a hip screen print (not sports related.) Now, if this guy can find any clothes that fit that description in the US, I give him kudos! And plus, I bet that if showed the description I just gave to any of his friends, they would promptly label it "gay." So, I would let the guy decide for himself. Option one is to buy clothes in Spain, and wear them and look like a hot spaniard. Option two is to dress like an American, and have everyone know he's an American, and have people make fun of him. But the reality is, that even when dressed in Spanish clothes, Americans still manage to look American. The question really is: Does your son want hot Spanish girls looking at his butt in tight jeans, or does he prefer they see it covered in baggy pants and boxers, neither of which are in style?
Ellenoregon, let us know how the trip goes, and what your son's reaction is! I'm very curious now!