damn dude, this is really crap...just because your "related" doesnt mean you dont have the authority to tell him what to do. your are his elder, and he should respect that alone! your doing a really good thing for him, i mean seriously, you've spent 1500 bucks on what is really HIS present, even if your not his old man, if someone did that for me i'd be goin bonkers! You dont deserve this from him and you should make that very clear to him.
BTW i babysit a 6 yo, who is one smart cookie with an attitude and when his mother punishes him, he just pulls the "victim" card, and says he'll stop behaving so badly, and that gets his mom all hopeful ,so she lets him off with a slap on the wrist...You just gotta maintain your ground, dont back down, once he realises thats how its gonna be, he'll soon clean up his act, he just needs firm boundaries and a clear understanding of what is right and wrong.
This is just my opinion dude, ultimately its your shop, u know how it runs best, dont take this as a stab at u, just friendly advice.