Around 18-24 mos, my DD started getting questions wrong on purpose, because she thought it was funny. You could tell she wasn't serious by the look she gave. We played along. This same kid, now 8, loves to try to provoke what we call "stupid arguments." For example, "That tree is orange." It's a personality thing, and it's just her being playful and silly, so we encourage it.
There are a number of other reasons why a kid might decide not to show what they know. For example:
1) Kids might feel like a performing animal, and decide they don't like the spotlight. In this case, it's best to stop asking them to perform in front of others.
2) Kids at age 3 often start to notice the transition from baby to big-kid, and feel insecure about it, which can lead to this behavior as an attempt to delay growing up. When our own DD started to express anxiety about getting older at this age, we dealt with it in a number of ways. One was to point out all the really cool stuff she can do now that she couldn't before. Another way we did it was to, on occasion, engage in an absurdly over-the-top performance in which we treated her as if she was a tiny, helpless baby. She LOVED that.
Ultimately, it's up to you to get inside your DS's head, to figure out why he's behaving the way he's behaving, before you can figure out the best way to deal with it.
