We just took our 3yo and 1yo to Sea World in April and one full day was plenty. If you absolutely must do everything, you'll need two days. However, we didn't feel it was necessary. We hit what we felt were the highlights and, on our way out, both kids fell asleep before we even made it to the parking lot.

We only saw three shows (dolphin, shamu, pets), but that was plenty. Our daughter talked about shamu, especially, the rest of the week. Still wants to be an orca trainer. Spent a fair amount of time (about an hour) toward the end of the day at the kiddy area, which is sesame street themed. Your son will be able to do all the rides there, as well as the wild arctic non-ride (note that this one has a real ride and a stationary version - we did the latter). There is also a big climbing net/treehouse all the kids seemed to like.