you don't need to do knuckle press ups on conrete for that... use a matt or something... the stress his wrists are getting put under is from teh stress of punching.

His body will adapt accordingly to that resistance by doing just that... punching... get him on a heavy bag

Press ups etc are good for develping the power and endurances of muscles... but the action of doing press ups on the knuckles isn't really relevant to what happens to the knuckles and wrist during punching