Hello Andy, welcome aboard.

Personally, whatever rep you buy him, it is likely to undergo an immense amount of scrutiny and unwanted attention supsect, so if you are going to go down the rep route, I would suggest def NOT an R factory or Omega one, given the amount of wannabe knowitalls out there at that age.
Maybe something like a Cartier Drive, JLC reverso or even a Tudor BB might be enough to put a smile on his face and keep the unwanted gazing eyes off, as most 21 year olds I know have little knowledge of anything outside of the R fac and Omega catalogues to hand.
Failing that, for a 21st maybe a gen Tudor or even a pre-loved R fac DD might fit the bill.