Sonny, I will get more details, but a group of friends went to Stone Island on our last cruise on their own and had a ball. You just catch a ferry a short walk from the pier, the ferry is only a couple of dollars each, and the restaurants were VERY reasonable--especially a place called Victor's.
Also you could take an open-air taxi to the Golden Zone for $10 each way and see the Papantla Flyers at the Aztec Theater; it's a show I think your grandson would like. And there are a lot of shops, restaurants and beaches there. The El Cid Resort lets cruise passengers use their facilities for free--and they keep the vendors off their property.
I'll get more information about Stone Island and get back to you.