For SeaWorld, look ahead at all the shows they offer and start making a plan about which ones you want to go to. Showtimes are usually online. Once you have a rough plan of what shows you want to go to at what times, you can plan around that. As a note, I believe most stadiums open up 30 minutes before the show starts, so I would recommend going a little early to find a seat that works for you (for example, you might want to sit closer to the front to see better, or sit further up to be away from the water). You can bring food into the outdoor stadiums so it's good to grab a snack and casually sit back and wait for the show to start.
Also check on the website what animals you and your son might be most interested in checking out. I believe they schedule some times for keepers and trainers to come out to talk about the animals and answer questions. Some animals can be a little tricky to find around the park, so if you want to see the beluga whales note that they are in the Wild Arctic area, and you kind of have to search them out instead of casually walking by them.
Speaking of planning, the park itself is pretty spread out and honestly not super organized, like how Magic Kingdom is. It's easy to get a little lost and turned around, or to just get too many steps in because you are zig zagging around the park. Having a rough plan will help with that.
The general rules to most theme parks are that people will naturally go into the first thing they see. In SeaWorld, that is Manta. If you are planning on riding the coaster or going into the walk through aquarium (which I recommend, it's a cool area) skip that as it will be packed. The penguin ride and exhibit is pretty popular and near the back of the park, so maybe try starting there first thing.
As a note, SeaWorld is an autism certified park so you might what to check on their website what services they might be able to offer your son. For example, I know they have quiet rooms you can visit if you need them, which are just an area that is quiet and low sensory if your son needs a break during the day.
