I'm post-Chiari decompression. I have heat and photo sensitivity for other reasons. I wasn't diagnosed until I was an adult, but I'll give you my recommendations based on my experiences.
1) If he has fatigue, you may want to look into being able to use his stroller (assuming you're bringing one) as a wheelchair (which means getting a GAC from guest services).
2) Sunglasses. The majority of the sun you're going to be exposed to at WDW is going to be walking between things. There's nothing WDW can do about that. If you do have him in a stroller, a sunshade would also help. If he'll wear hats, a hat with a good brim would be another thing to try.
3) You can buy cooling bandanas. They're more affordable than cooling vests and will be colder than just using wet rags. I'm fairly certain REI sells them, but they're available many places. Last I looked they were under $20.
4) Misting fan. These are fans that have attached water bottles. As the fan turns, it also lets out a mist of cold water.
5) Make sure he's drinking loads of cold water. You can get cups of ice water from pretty much any CS (should be free AFAIK), or bring a cooler with your own. Water is going to be better than anything else because it won't dehydrate him.
6) You didn't say whether he has any skin photosensitivity (whether he burns easily), but at least 30spf suncream (once you're past 30spf there isn't a huge difference in efficacy between different strengths, but a minimum 30 is probably good).
I'm not sure when you're going to WDW, but it may be worth looking at clothing that has maximum coverage, but is flowy so that it blocks most of the sun, but isn't so tight it gets sticky.
You may find it most helpful to go to the parks for rope drop, spend a few hours, then go back to the hotel for a nap and maybe some pool time during the afternoon when it's hottest and sunniest and then going back in the evening. That will reduce his sun exposure and also build in a rest time, which I'm sure he'll need. It's easiest to do if you stay on-property, but is possible if you're not too far off property. You could also use First Aid if he needs to lie down and you don't have time or can't get back to your room for whatever reason.
It's also worth checking with his neurologist/neurosurgeon for any additional restrictions. He probably won't be tall enough for rides that have height restrictions so being jerked around is probably less of a concern, but his doctor may either have more suggestions relating to his specific symptoms or some sort of restriction that I'm not thinking of myself.
