So later he wanted to ask me about that and what it was. When we talked about he didn't seem to get why he couldn't say "I am great at Math" because he is. DH said to him "well you could say I really like Math" but DS was like "but I am really good at it...why can't I say that?" He has a point. Just because he is great at it, doesn't mean he is implying that others aren't.
We've explicitly told DS5 that it's a social convention that you don't usually tell people you're good at something. We've told him it's fine to tell us he's great at something, and fine to tell other people he really enjoys it, and (harder for him to grasp) that a good response to someone saying "wow, you're really great at X!" is "well, thank you, I do really enjoy X". He's pretty comfortable with the idea that social conventions don't always have to make sense :-) Actually, though, although we've made a point of discussing it, I think he'd basically picked this up already.