How about a kids play tent? It's the season for tent. He can keep all his toys in the ball pool and crawling through the tunnel of the tent to exercise his arms and knees. If he can invite friends to play with him, they can spend hours play in it. Tent can inspire kids' imagination as well as enhance their social and emotional development. https://www.amazon.com/dp/B01N7SQJUP Ideal for indoor and outdoor play.