Free: Contests & Raffles.
get a rack of ribs,put in large ziplock bags. have him back off it to your set shooting distance. shoot and check you penetration. Think a pork rack should be good size for a deer, beef would be elk. that way you can figure out your distances he can penetrate.
My Wife only shoots 24.5" So she is only getting 20-25 FPS more then your son at 22"She does shoot a little more poundage (46#) to be exact, and she has shot clean through several deer and killed a Zebra with that setup..As long as he is accurate and has a sharp broadhead I say no problem!