What are all of his friends doing this summer? Are any of them doing a daycamp? That's the way I got my kids to go to them. Then I arranged playdates after the camp where we took turns and sometimes they would go to a friend's house and sometimes they would come to my house.
Another option would be to talk to one of his friend's mom and offer to pay her to take him a few hours a day.
That's all I can think of.