My youngest son was into Army stuff at that age and I did a backyard Army party. I made an obstacle course out of straw bales, used old tires to run through, etc. like a basic training course. I just served hot dogs, lemonade and cake. I'm sure you could find some activity ideas for whatever your son is into.
Another idea is to go to the beach at Walled Lake. I haven't been there in a few years, so I don't know what the admission price for the park is, but I think it was $4/person then. It's nothing fancy, but the kids could swim and then you could cook out on their grills. It's a nice little park (free for residents of Walled Lake). If you can't find info, try the city of Walled Lake.