It's a good time of year to go (as I'm sure you know), so the crowds shouldn't be too bad. Make sure you don't go on a weekend, as a lot of locals visit then. Also, if you go on a slightly overcast day, most people avoid the parks, even though at this time of year it's not too cold.
The only worry with this last part is that they close the parks when there's a risk of storms. You would probably want to prepare him for this possibility, and make sure he knows he has to get out of the water
straight away if told to do so. I would, however, recommend you don't head straight for the buses, as everyone else in the park will also be trying to do this

. There are food outlets at both parks, so you could make your 'plan of action' include a snack at one of these, after you've got changed.
A lot of preparation will probably help you guys a lot. Take a look at maps and photos together; talk through the things he's likely to encounter (such as lines; people bumping into him; having to wait at the top of slides until the CM says he can go, etc); make a rough plan for your visit; and anything else that might help him. I'd suggest you plan for the visit to be a short one the first time, and if he copes well with it maybe visit again.
Below are a few ideas that might help, but you take them and play with them as suits you:
Establish a 'base point'. There are lots of deck chairs around the wave pools at both parks, so if you take a beach towel you can claim one of these. Try to pick one that is a little away from the main paths, but close to something recognisable. That way he will have a 'safe space' to retreat to, and also a staging post for you to meet at if you get separated.
Start on something that won't have a line for it; the creek is a good way to start off gently. Do make sure he understands that there are points where the rubber tubes bunch up, and everyone is close to each other. If he's in a ring, it will provide him some 'protection', but it will also make it harder for him to get out of the jam. These jams are usually short, and clear out again by themselves, but he needs to be prepared for them. Also, make sure he knows that there are various features along the creek that squirt you with water, such as waterfalls and water guns. If he would rather, he can swim through the creek, but you do sometimes get the rings pushing in on you from all directions, and it can happen suddenly.
The wave pools can also be accessed without any waiting. At TL the wave pool is bigger, and so are the waves. To start off with, don't go in more than half his height; when the waves come this will be deep enough to sweep him off his feet! The waves are forceful and large, though you do get plenty of time to prepare for it (just listen for the screams from the deep end!). There is a chance his head will go underwater, and the currents can pull quite hard, so make sure he knows to take a deep breath before the wave comes, close his eyes, hold his nose and face his back to the wave. Tell him not to panic if he goes under, just to hold his breath and try to find the floor with his feet; the waves pass quickly and he'll be back within his depth soon. Often people will get swept forwards into you, so he has to be ready for strangers bumping into him. Also, be warned that the floor is fairly rough, especially at the shallow end, and I've skinned my foot on it.
The wave pool at BB is much smaller and has constant bobbing waves. There are also inner tubes in the water, that you can use to float in, but there is usually a wait for these. If you start off just swimming, then you can grab one when it comes available. The crowds in this aren't too bad, though he may still get people bumping into him.
Both of the wave pools have sandy beaches around them. How is he with sand on wet feet? If this causes problems with sensory issues, there are hard-paved paths into the water, though there is usually some sand on them. There are plenty of places to wash sand off feet, so you can line up with them and dash through the sand quickly.
If you want to try slides, you could start on some of the family or two-person ones, so you can go down together. On the single riders, you might want to go down first, tell him to wait until the CM says he can go, then meet at the bottom. For these there will be lines, and there are no Fastpasses in the water parks, so he'll need to be ready for these. I suggest you break up the slides with breaks in the creek, wave pool or having a snack.
Hope some of this helps, if you have any more questions, do just ask! I hope you guys have a wonderful trip, and that your son copes with, and enjoys, the water parks.
