I don't think it's a matter of intelligence as much as personality. That being said, it IS a sign of at least a basic level of intelligence to be able to put together all the clues and logically deduce that these are people in costumes, despite having all the adults around you tell you they're "real". And then, it's entirely a matter of how compliant and/or easy-going the child is, whether or not they feel the need to point out their discovery to the adults around them. Or whether they decide to just go along with the game.
I suspect many kids know in a logical way that they're people in costumes, but just like many adults, they really don't care that much. There's nothing "only" about pretend - it's very real, when you're in the moment!
This shouldn't make you sad. You should be pleased he has demonstrated strong observational skills and an ability to think critically.
And why should he not still LOVE the characters, now that he knows they're people in costumes? He can still love them, unless your sadness ruins the game for him!
Why would you expect them to stop wanting to see the characters, just because they've figured out how it works behind the scenes? Many adults still like seeing characters! It would be sad if you decided not to take them to see any of their favourite costumed characters any more, just because they know they're "not real".
"Real" is overrated, in my opinion. Especially at Disney! If my kid had said to me there are "people under there in costumes", I would have said, "Of course there are, now which characters are you most you looking forward to meeting?"