But at that point it would be too late for the acceleration of gravity to matter all that much? And if he makes it heavy enough to matter, then at that point he'd probably throw himself horribly off balance...

Force = mass times acceleration (with acceleration being mps squared).

Gravity does not increase mass. Gravity CAN increase acceleration, which admittedly is the bigger part of the equation, but you need time for that to build up. There are ways to make it work, just requires some thought. Definitely won't work well with regular punches though.

A flying hammerfist however......