If you are looking for things to help out with braces: sometimes they poke into your mouth and you need pliable wax to put over them or Orajel, a Water Pik helps get the food out and with the sore gums, though the Ortho doesn't like it, I chewed sugarless gum after I had mine tightened because it helped the soreness go away (just like a mild workout helps muscle soreness ease), perhaps a bottle of pain reliever to take after the tightening appointments.
Those all help get through the worst times.
Not sure if you meant other 'stuff' for a 15 yr old to take his mind off of them?
As a parent who's paid well over $12,000 for palate expanders, braces & retainers for my 2 kids, I'd be <ahem> more than a little upset if a relative told my kids to chew gum even if the ortho didn't like it.
Braces today are not made of the same material as they were even 10-15 years ago. The metal brackets are softer and can bend while biting into an apple or even a really hard roll. Happily, they no longer place a whole metal band around each tooth--the brackets are glued directly on the teeth (less "metal mouth" effect).
The list of no-no's means nothing sticky, chewy or hard. So, no popcorn, whole apples, peanut brittle, tootsie rolls, starburst, gummy anything including vitamins, skittles, gum, snickers bars, twix, rice krispie treats, marshmallows, etc.
Do you see kids w/ braces eat that stuff? Yes, but if their parents don't mind spending more $$$ to have broken brackets removed then reapplied, that's not my business. I just wouldn't be the one to give it to the kid.
You're safe w/ plain chocolate, ice cream, pudding, pie, cakes etc.
To the OP, you don't mention what your nephew's teeth look like (crowded, overbite, etc) but my kids hated to smile before they got braces because they were very self conscious of their teeth. So, just before my kids got theirs, I searched the web and found some sites w/ before & after pix of kids who had braces. Seeing the difference that the braces made in someone's appearance was huge for them. Even better, the kids in their 'after' photos looked so much happier and confident. For my kids, it gave them incentive to take care of their braces and their teeth. Plus, the orthodontist wasn't just a mean person who made their teeth hurt. She/he made kids look & feel more comfortable w/ themselves by giving them beautiful smiles.
Just some food for thought...
. BTW, you sound like a very thoughtful aunt to be so considerate of your nephew at this time.