For managing visual chaos, I think for me my biggest strategy for that has just been to learn to self-monitor. If it is becoming too much for me, I go somewhere less chaotic and small to calm down. Usually for me that means a corner in a gift shop or a more isolated bench (like in critter country or the path from fantasyland to frontier land or the covered bridge area near small world and Matterhorn). If it gets bad enough and I can’t get somewhere relatively quickly, I’ll close my eyes and have my wife lead me somewhere better. Sunglasses are a must on most days, but only my prescription polarized ones. I also have a tendency to be glued to my phone and not look up, but rather hold onto my wife and let her lead me around. I avoid the Astro orbiter area of Tomorrowland as much as I can and the entrance to Adventureland since those are some of the worst spots for me with all the movement and people going every which way. I also avoid fantasyland in the afternoon since I have a hard time with it at that time of day. Main Street can be a problem, so I either take a ride vehicle in the morning if there are a lot of people or I cut through the shops.
At 4, that is going to be a lot more challenging. Does he let you know in some capacity that the visual stimulation is getting to be too much for him that you’d be able to go somewhere else to take a breather? Sunglasses may or may not make the situation worse in some instances. I know with my really dark sunglasses that they can actually make things worse for me when they obstruct my ability to clearly make out what is in front of me. It screws with my ability to process more. I’m with your son on no headphones with sunglasses though lol. It’s really uncomfortable have the sunglasses pinch behind your ears from the headphones. If you give him the option of headphones or sunglasses, will he pick one?
I would definitely say knowing what happened your last visit, build your itinerary with LOTS of breaks. Do a few things, and then go find a corner of the park for him to just chill and watch ducks or engage in an activity he finds fun. Spend the afternoon back at the hotel if you can. Use maxpass so you’re not having to walk all over the park back and forth to retrieve FP. If lines are challenging for him with all kinds of people packed around him and he does better waiting elsewhere, try a
DAS pass or getting a tag to use your stroller as a wheelchair so he can stay in it in lines.