I was flying Jets at 21..course mates were as young as 18
Some great advice ..First class medical...join french flying club and get a ppl. Make himself available for any work including with a broom in his hand.
Join a french gliding club and do a course..they are subsidised for youngsters..do enough work around the field..get tugging.
Meantime study..anything that he enjoys ..which will indicate commitment and give him a fall back career...
RAF requires UK residency for 5 years, exceptional circumstances 3...
Good luck..took me 5 years after my first knock back..then plan B after I lost my medical at 45..Still flying at 72.