Check this out.
https://www.cabelas.com/product/GAMO-VARMINT/2261963.uts?slotId=0
Smoking deal on something that is accurate and fun to shoot. Since you'll be supervising this is absolutely the route I'd go. Most of the pump style bb guns are like shooting a carnival gun IMO. If you want him to develop good shooting practices, teach him with something that shoots good.