I'll disagree with those that say it's still "hot and humid" at night. However, hot is very subjective. It's still humid...it's summer in Florida, it's almost always humid. Hot at night, not so much IMO. Once the sun goes down, I think it's fabulous. Usually in the low-mid 80s, without the sun beating down. If it's a "cool" day in Disney, it could even be in the 70's at night.
Anyhow, yes, you could definitely do an evening tour, I don't see why not. We don't do exclusive evenings during our summer trips, but we do some. Would the pool be the best idea if your son has issues with daytime? Sure, you're in the water, but the sun is beating down. If he can do that, maybe you could get to the parks somewhat before 6pm. There are endless options to do that are pretty much all inside in the a/c. Things like Philharmagic, CoP, Bugs Life, Dinosaur, Muppets, SE, etc.... The options are even more if you have FP+, for example Space Mountain or Star Tours. The line there can stretch outside if you don't have a FP+. But if you have one, you walk right inside and stay inside.