I think they will let you go as long as the suit fits, and if they don't I am sure they will refund your money. If he really looks older I just wouldn't say anything.
You don't say where this is, but if he is a good swimmer if he can't snuba then he can probably snorkel (if he gets turned down) anyway.
And if he gets turned down don't tell them you saw anything about an age limit when you booked it. (plausible deniability)