Ruger 10/22. Simple, easy, reliable, lightweight, stocks can be had for cheap and he can grow with, and into it. Started my kids with one of mine and a red dot at about 6. One round in the magazine at a time, EZPZ. They both built their own AR15s at 12. Kids are capable of more than most think. As long as the safety is totally ingrained, get them something they will enjoy the rest of their life, not use a few times and outgrow.