I would start him off with a 16ga shotgun,or a 243 rifle. the 16 ga my wife had kicked a lot less than the 20ga. I have,(3 of them.)you can get #1 buckshot for them.only #2 for the 20.if buckshot is not legal in your area,it will do great with slugs.
the 16 will let him grow with it.the 243 is a great little ("no recoil"
gun. I have shot one for the last 20 or so years.
I am not a hunter I am a whitetail population reduction specialest
remember keep your back to the sun, your knife sharp, and your powder dry.