I live in Tampa, but it's been a year since I've been to Busch Gardens. There are plenty of nonoller coaster rides to keep you busy for a day. If you like animals you can explore lots of different animal areas and mixed in are rides the two of you could ride together and few he could ride alone. Keep in mind that it's already in the high 80's here. So pack a bathing suit and flip flops with you to the park. There is a kids area with fountains he can cool off in and there are three rides where you can get wet. There are also some fun shows which you can schedule for the afternoon when it gets hot out and you need a rest.
They will check your bags before they go in. They allow water bottles and some light snacks but no coolers with food. The food in the park is actually pretty good. We really like the BBQ restaurant.
School is still in session here for a few more weeks so I would expect that during the week it won't be crowded but you might run in to school groups.
Other ideas for Tampa -
Adventure Island. Water park right by Busch Gardens.
Lowry Park Zoo - very nice zoo, some foutain/water play areas to cool off, outside the zoo is a park where you can picnic. If you have a pass to a local zoo in your area and check and see if the pass is good here too. Lots of zoos have reciprocal relationships that allow free admission.
Museum of Science and Industry (out by Busch Gardens) - Science museum that also has an IMAX theater. If you have a local Science museum and have a family pass check to see if you can visit here for free on your pass.
Florida Aquarium - this only takes a few hours to visit. There is a great splash/play area out back that overlooks the channel/cruise ship area. You can sit out there with a drink and watch him play.
If you aren't looking for a full day thing there are lots of places to do trampoline jumping, bowling, ice skating, etc. If you like baseball the Rays might be playing in town that weekend. We also have several minor league teams in town which are super cheap to see a game and a lot offer fireworks for free on the weekend.
I hope that helps! What area are you staying in?