870 I would say to have him lean forward a bit and the recoil should take him back slightly,short of a 410,20 ga is only slightly more bam.Don't make him or he will shy away from you and become distant later on.Rave about his accomplishments and don't focus on his weaknesses,He will become a stouter person 1 year from now and you can never go back.You can get back to the place but not the time,enjoy the youth cause it goes away in a year or so and won't come back.I just wish I could get back in time and fix all my faults,but that won't happen,so enjoy what you have,your a lucky person to have a son in the magical years,He soon will become a man and you will have fond memorys.Drop-Shot