I have been to WDW more than 15 times since I was diagnosed with MS including going in June, July, August and September - all the hot months. I have also been in May, December and January. My preference is early May. It is a nice balance with warm weather but not the intense summer months heat and slightly lower crowds.
I also have issue with heat causing problems and the best solution I have come up with is staying well hydrated. Drink water, drink water,
drink water. Every quick service location will give you free ice water and if you ask nicely, most will give you the bigger cup or two smaller ones.
In all the parks the stores are air conditioned so if he starts feeling too warm, head for the nearest store.
At the Magic Kingdom and Epcot all the shows and movies are inside in air conditioning. At Hollywood Studios the Beauty and the Beast and Indiana Jones shows are outdoor theaters but all the other shows are air conditioned. In Animal Kingdom the Safari, the animal trails and the bird show are outside. Both the Lion King and Nemo shows are inside with air conditioning.
I try to alternate shows with rides in order to limit the amount of time I am out in the heat.
I have a personal, battery operated fan on a cord that I can wear around my neck to blow air up towards my face. I can also use a mister fan and cooling bands or towels like these
http://coolbandanas.com/.