RE: 20 or 12 guage?
It really depends on the your son. I have two sons the oldest at 11 was shooting a 12 gaand the youngest at 12 is still shooting a 20 gu most of the time although thisspring the youngest did shot a 12 gu turkey hunting without any problems and harvest a 3 bearded jake. Take him out with both guns and see which he handles the best. My oldest son was much bigger than my youngest at 11. My oldest is 15 now and is 6'4" tall and weighes 210lbs pretty good size boy for his age.