I usually just say thanks to the person. DS is normally so involved with whatever he's doing at the time that he doesn't notice at all.

Because he has two step brothers around the same age I guess he knows that he can do more than them but to him it's not even a topic of conversation. My step son has asked me a year or so ago why DS could read and he couldn't but DS has never mentioned it. I guess he thinks it's just normal.