We have this with ds(6) and dd(5). Ds will often snigger if she struggles to read a word that he couldn't read at her age either. We have got to the point where we do pull him up on it. I will remind him that I can do his homework quicker than he can - he says that that is because I am a grown up and then I tell him to think about what he has said and how it is relevant to what he has just done to dd.
We have also spent a lot of time telling him that whilst it is great to be good at things, other people who might be struggling with things don't want to hear how easy you find it and how good you are at it. We think about something he has struggled with and then I say "what would you have liked me to say to you there, "ha - I'm really good at that, it's easy" or "yes it can be tricky can't it".
I have to make him see it from the other person's perspective.