Start training kiddo on walking. Miles and over terrain to build the legs up.
Hit the parks early, we rode/viewed til about 12, had lunch, one more ride then back to hotel. Swimming tired ours out more than walking so we enforced a 2 hour nap. It's all dependant on your child. Mine (7, 5 and 3) still nap as soon as we get in the car if we do a full day at a themepack.
We went back to parks about 5 for dinner, took the buggy/stroller with us and a light blanket. Mine were all asleep by the time firework time came around, so I cushioned their ears with the blanket and watched from Emporium corner kerb. Took advantage of shopping after fireworks or headed out depending on how busy the parks were. If yours is still awake, pop them in their stroller so they feel in a familiar environment. My 3 year old still does this in the cinema, it's too noisy for him sometimes! He likes to feel cocooned
Can you get some back garden ones to gauge a reaction, or take him to a display. But to be honest, Disney fireworks are so not the average huge bang frightening displays you get outside of Disney (if that makes sense!). Because of the music and visuals, it's not all about the bang.
Book character meals. It really helped my kids stay still so the grown ups could eat at a more leisurely pace.
Accept that at the very moment you turn up for your FP your child will need to pee or poop or meltdown or be hungry or fall over etc etc and you have to switch plans. That is probably the most important part. Disney does weird things to kids and can turn them into unknown variables.
