Many kids follow the rule
"Ask a silly question, get a silly answer."
Meaning if it's an OBVIOUS question... they take it as you initiating some sort of game... and participate (saying something silly in return).
Others just refuse to perform. Which is totally natural.
My toddler did this to people who asked him "baby" stuff... my 8yo does this to my mum when she asks him math questions that he learned years ago. She was actually really freaked out last week about how "behind" he was in math, because he wouldn't do times tables with her. Until she saw that was because he was about 4 years beyond times tables. Kids sooooo often go in one direction or another (What color is the FIRE truck? Blue. ... or just out and out "Not gonna answer that one.")
I think the question is HOW do you know he knows the answer? If he's told you in the past that it's probably silly answer or refusing to perform. If he *hasn't* been able to verbalize the answer at any time in the past.. you might want to seek out an evaluation.