depending on how much you want to spend, i believe that B&H in rocktown has an EASTERN TRAILDIGGER which is actually a 24" bike but is the style that you are talking. designed around kids and jumps. not really a mountain bike per say but not a 20" bmxer either. (close though). then i would say next in line would be maybe the fisher mullet. it is a geared bike and a rider in the club rides one regularly, trails and street. likes it from what i know.

without going"over the top" it is going to be hard to get him on a true "jumper" as they are super pricey even in the lowest stages. another option is ebay. i know, i know support your lbs. (i did) i personally bought a bad a** jumper (in my opinion) comlete off ebay for about $280 plus shipping. there's no way possible i could touch this bike new for under a grand........ just know what you're looking for so you dont get taken. find a friend if you need to.

good luck in your search.