Obviously, as I don't know your son, I don't really know what would suit him best, but I'll throw out some suggestions on possible ways to tackle the parks and you can see what sticks.
I would suggest you start on the lazy river. That way he's got a fairly gentle introduction to the water park, some time to get used to the theming and the place in general, without being too overwhelmed. It's also got some nice talking points as you go around, such as a snow covered house with a sick alligator inside (he sneezes so hard that water comes out of the chimney), which are funny without being 'too much'. Also, if you're at the park from rope-drop, I would think the lazy river would be comparatively quiet, because most people rush straight for the thrill rides, and leave the calmer stuff for later. They have kids rings available that have a 'seat' in them (basically a rubber ring with plastic filling in the hole in the middle) which you could hold on to to make sure you didn't get separated, or you could share a double ring (figure of 8 type shape).
It might also be good to balance out the high stimulation stuff, such as the kids area and the family raft ride, with calmer stuff, like the lazy river and the bobbing waves. If there is a line for the family raft ride, you can try and distract him by looking at the view: because it's so high up, and Florida is so flat, you can see for miles and miles up there. Also, the queue moves fairly swiftly, so he shouldn't need to be distracted for too long. If you are a group of two (if you decide to split up at the park), they may ask you if you would be OK to share your raft with someone else. The advantage of sharing is that the extra weight makes it go faster, the disadvantage is that your son may not be happy sharing with strangers.
Some of the rides have height restrictions, so you might want to check them out before he gets excited about an attraction. The height restrictions are at the bottom of each ride description on the
Blizzard Beach section of the Disney webpage (that link is the UK website, but the information should be identical on the US one). If there is no restriction listed, then that ride doesn't have one. I think the best thing to do would be to familiarise yourself with the park first, then mentally walk through a day there, finding any potential problems and, if possible, avoiding them. There are photos or videos of most, if not all, attractions in the park on that link, so they should help you find 'trouble spots'.
EDIT: Just in case he really gets into water parks, here's the link to the
Typhoon Lagoon page.
I think it would be good to make sure you get into the water quickly, with the smallest risk of a meltdown due to delays (changing, getting towels, getting lockers, etc). If everyone is wearing their swimming costumes under their day clothes, you can just pull off shorts and T-shirts, then be ready to get in the water. Then one of you can take the kids into the water, while the other adult gets the lockers and towels sorted. That way your son doesn't have to wait in line any more than necessary. I'd also recommend you all wear old T-shirts over your swimming things, to try and prevent burning (as well as water resistant suncream, of course). It's not really a HFA-specific tip, just one that my family have picked up. A burnt kid is not a happy kid, and will probably increase the risk of meltdowns!
Hope this lot helps a bit! If you need anything more, we'll do what we can to answer questions!
