not paid by hours worked but per week
What do you mean? He must be being paid the National Minimum Wage (otherwise he needs to report it and look for somewhere which treats its staff legitimately). Or do you mean he's on an Apprenticeship? In which case he'll have known the rate before joining, presumably?
Needs to negotiate less hours and or more money without getting sacked or seeming to not want to progress
I'm struggling to see what his bargaining chips are though, especially since he's so new. 17 year olds on minimum wage aren't paid well (presumably you're supplementing his living costs/he still lives with you), the job market for unskilled positions is fiercely competitive...
Is he on an actual chef training scheme? i.e. he has a realistic plan/understanding of how to become a chef? In which case this is a temporary issue (low pay/long hours/etc) rather than a career problem.
Also, 3 weeks isn't long to stick in a job, even with those hours - most entry positions have tough hours, environments, and chefs typically work stupidly long (and unsociable/not family friendly) hours even when qualified/well experienced