I'm sorry, but I have to agree with some of the other posters.. This is
not a good horse for any child, especially a non-horsey one. You should discuss that
immediately with your uncle. Refund the family friend their money and take that horse back! You or your uncle sure could face a lawsuit since you knowingly gave a wild horse to an inexperienced youngster.
If you can't afford a trainer or don't know how to deal with an abused/unhandled horse, do the best thing for the horse and all parties involved, and donate it to a rescue that specializes in that. If you feel bad about taking away the kid's horse, then help him find a nice dead broke older horse - which is what he should have right now. Winter is just about here, and most people are trying to give away their older guys to good homes for free!!
Be stern. Tell the parents, "This horse is too much for the youngster. If you do not get rid of it, he
will get hurt or killed." Especially since he is anxious to start riding... what if he sneaks up onto that horse's back one day when no one is looking? Yikes...
Sorry if my post comes off as harsh, but this is not the same situation as trying to rehab a feral kitty. This is a 1000 lb+ animal that needs very experienced handlers who know what they are doing. It is in the best interest of the horse, the family who has it now, and you!
I hope that you at least contact a rescue group in your area. Tell them your situation and ask if there's any way they can help. Maybe they will even trade for a well-broke horse they're looking to rehome. You never know unless you ask!