I wasn't going to respond since I don't want to be one of those "this is how you should raise your kids" people but I'm going to anyway

What you might consider is doing the entire Living Seas pavilion so your son can see how all things in the water live. The manatees are at the end in the walking around part so you can enter through the exit and just see those but it might make a more lasting impression with the entire attraction. Just a thought.