I believe that dressing is a very personal issue, however, I can answer some of the MM questions
1) Will he be denied entry to some restaurants because of his attire?
Yes. In some restaurants he will be denied access or he will feel like an alien.
2) Will potential pick-pocketers yank his pants down around his ankles in order to rummage through his pockets?
Not probable :p
3) Will he be targeted as a foreigner/tourist?
Without doubt. That kind of dressing is extremely rare in Spain (though not unseen).
4) Will he feel out of place or more comforted in his unique-ness?
That depends on him
As I have said, he will be targeted as a foreigner for sure, and he will be the only one dressing like that in miles around.
5) Will he return from his Spain visit wearing the latest "cool" Spanish fashions and impress his friends?
Again, it depends on him and what he likes.
6) Will he spend the entire trip in the hotel room taking advantage of the free high-speed internet access (i.e. GoMadrid's son at Hostal Adriano - hehehe.. )
That would be a crime! There is so much to see and to enjoy in Madrid for a 15 years-old guy.
7) Will his clothes turn off the girls in Spain?
Maybe, or maybe the uniqueness will attract some girls. It's difficult to say.
Perhaps the best would be to carry a variety of clothes so he is able to choose what to wear once he sees how people dress in Spain. If he feels comfortable being the only one wearing big jeans and cargos so be it. But suggest him to bring some more clothes to avoid something as fool as this ruin his trip
Fernando