We are so negative on here sometimes it's painful. Yes, Jay is being held over a barrel. Yes he's being shafted. Yes, it's illegal to pay apprentice rate unless as part of a formal apprenticeship agreement. BUT - someone has given the man a chance, and is paying him SOMETHING at least. Better than those poor sods offering to work for free, no? Would we be happier if Jay had NO job? Sure we'd have the moral high ground that we'd stopped him being exploited but you can't eat morals.
Best of luck Jay. For what it's worth I'd have done the same as you (did something similar in fact). Learn, absorb, progress and demonstrate to your boss that you're a valuable asset. The financial rewards will come. Do suggest, as soon as you are earning him money on your solo days, that he pay you more on those days.