The thing is, he's 15 so there's no min. wage therefore you simply haven't got a case against him. As long as he doesn't work more than 8 hours during the day and has a 1 hour break every 4 hours then he can pay him whatever he wants.
Even if there was something to report this guy to, the fact that he only worked for this guy for 2 days doesn't exactly bode too much in his favour for making a claim. Not exactly sounding like this kid was too much good at the job.
Just be glad he actually got paid as an awful lot of people wouldn't bother.