It depends on which kind of animals he likes the most.
We did the Dolphin Up-Close Tour a few years ago and loved it. In fact we're thinking of doing it again when we go this year we enjoyed it that much. I think it was around $45 for adults and $25 for kids.
That particular tour involved lots of interaction with the Dolphins as each group was split up into smaller groups and every group got their own Dolphin to interact with (touching and teaching hand signals to).
Originally, I was going to recommend the Beluga and Walrus Up-Close Tour but you have to be 8 years old and over for that tour. I've never done the Sharks Up-Close Tour but maybe he'd like that one?