First word of advice. Make it FUN for him. Getting a ten year old to sit still for any length of time can be an impossible task. Kids get bored easily. A kids idea of fun hunting is often alot different than ours. Keep in tune to the child and if he seems to be getting bored switch to something different to keep his interest up.
If your son has never shot a gun before, start out with a pellet gun or a .22. After he is comfortable with the non kicking crack of a .22 I would try a .410 shotgun. Let him shoot a few rounds a day for a couple of weeks and if he seems to be able to handle it,, move him up to a 20 guage. A 20 guage with #5 2 3/4 inch shells is what I killed my first turkey with when I was 15.