Sabot slugs suck. All I see is dollar signs when sighting in and foster slugs have always been good to me out to 100 yards. I'll never own a rifled slug gun. Depending on his age/size I'd recommend a full size gun and even if you have to buy an extra youth stock. Kids grow fast. I'd go with an 870 (parts and stocks are readily available). I also saw a Charles Daily Combo for $300 on sale on a cabelas ad the other day. IIRC, the Charles Dailys are just like the 870's.
For scopes go with a red dot, 2x or a 1.5-4 variable. Maybe even a 2-7x. But straight 4X is too much for tight woods hunting. Low power scopes rule for quick shooting.
LK