K.W.
Blue Man Group is awesome! (I've seen them in Vegas, Boston and their touring tour--lots of fun and family-friendly (your older kids will like it too).
If you want to splurge--Discovery Cove is an awesome experience, but $200 for the Dolphin Experience ($100 without) for FL residences.
Other than that--does he like any sports team or collect anything?
What about a watch? Is he picky about that kind of stuff too?