If you are in the bethesda area, One Life Fitness in downtown Bethesda has an indoor basketball court. High school and college aged play pickup basketball there. i am not sure the exact days/times b/c adults also play pickup there too. Sometimes the adults and kids play together but I do think the kids play at specific times.
the front desk can tell you more, but you'll have to buy a membership for him. I think it's free for kids of members as long as the kid is 13 and under.