15 year old boys are always hungry. I think chocolates, or better yet, donuts! would be appreciated!

....and I think they resolved this pretty well a couple of years ago.

i think i'd like a gift certificate to a steak house. (don't know why I am hungry now).

I guess the performance has been long gone.

Or maybe, now that he is old enough to appreciate flowers, you can send him a belated bouquet?

I don't think flower-giving for men after performances is the norm here in the U.S., as much as in other places.

I do remember the banana split my parents treated me to, after my first recital...

Laura,
my family always took me to a restaurant for a late dinner in order to celebrate the event, discuss the performance, meet with interesting people we invited to the concert and replenish the energy. It's a wonderful thing to do, sometimes it means much more than any other material gift.
When I'm performing in other European countries I usually receive flowers and sometimes a cocktail party in order to meet with the guests of the organizer.
Perhaps you could combine some of the interesting suggestions by the others and add something yourself.

Cloth-bound Henle sheet music

Okay, what is he, 17 now? No more rifle, it's time for a 45 or 9 millimeter.