The options, of course, are to sell "stuff" or sell "services." All without placing too much weight on a young back; the work needs to be suitable to abilities such as strength, maturity, coordination, ability to focus, etc. Parents sometimes overestimate; and when there's power equipment involved, that's bad medicine.
Random thoughts:
If you get along with and trust your neighbours, broach the subject. Sure they do the work themselves right now, but they might be willing to give the young lad a casual job. Especially if it's couched like "he wants new drums, but we think it's good for him to learn some practical skills and earn part of the money himself; it teaches budgeting skills and helps him build a good strong work ethic." Country people respond to that philosophy and those values, and may step up to support it. And maybe there's a senior who would appreciate both the help and the company.
Of course you can't drive him all over the place. But if there's a good farmers' market in a nearby town, for example, there may be options. Busking with minimal drums and a couple of friends (hint, play for the crowd; and big sign saying 'fundraiser for new instruments')? How about sharpening small kitchen knives for little old ladies? Even giving 2-minute drum lessons (sign me up!) to passers-by. One-on-one computer lessons for folks at the seniors' lodge (donations please)?
One thing I've learned: once you're out there and visible, all sorts of opportunities turn up; things you never would have thought of sitting at home.
Good luck!
Doug