You've had some great advice already, I'll just add my tuppence to pot.
Firstly, look into the GAC, it could be a real life-saver for you guys (see the FAQ at the top of the disABILITIES board for more information).
Secondly, as has already been said, try to avoid being out and about in the middle of the day. If possible, head into the parks first thing in the morning, explore for a while, then when it starts to get too hot, head back to your hotel. You can swim, read, watch TV in your room, whatever you fancy, just stay cool. Then when mid-afternoon rolls around you can head back to the parks and play some more.
There's loads of air-con at WDW, so take time to enjoy it. Watch a show, stop in a restaurant for a drink or a snack, wander around the shops etc.
I think Animal Kingdom will be the hardest park for you guys as it's HUGE (it could apparently fit the other three parks within its borders comfortably) and doesn't have as much air-con as the others. Try to hit this one on a cooler day, and preferably at a cooler time (such as first thing in the morning or later afternoon). Be aware that the park closes fairly early in the evening (about 6ish if I remember correctly) so don't expect to do too much later in the day (unless you go to evening Extra Magic Hours).
One tip I've picked up from these boards is to carry a wash-cloth in a zip-lock bag, that way your husband can dowse down regularly to keep cool. Just remember to carry the suncream with you and to re-apply regularly if you're doing this.
Small battery operated fans might also help keep him cool. The misting fans are particularly useful and can be re-filled at any of the water fountains.
Also, be flexible with your plans, that way you can arrange your schedule according to the weather and temperature outside. I've known people who run their holidays like drill Sargents, but I find going with the flow to be a much easier way of doing things.
One last tip, when you're at the water parks, wear an old T-shirt (one that you don't mind getting bleached by the chlorine). We always do it to keep the sun off, but it's also very good at keeping you cool when your out of the water.
Other than that all I can think of is the normal stuff like take care of yourselves, drink lots of water, don't over-do it, etc.
Good luck and have a great trip!
