To the best of my knowledge, he'll have to go through a USAF Pilot Recruiter. There's no established process for that (there used to be a blue to green process, which allowed surplus Navy/AF folks to transfer to the Army, but not the other way).

If he still has an AD obligation to the Army, he will have to get a conditional release from the Army (basically army releases him on the condition he enters the AF). This sort of release is somewhat routine, and would probably be granted unless he's some high-demand/low-density capability like SOF.