I like the answer "In what way?".

I might have said "What makes you say that?" or "Why do you think that?" if my son said this to me, looking for more information on where he was coming from and what response he was looking for.

But given that he is an avid RPGer, I'd have probably just said "Possibly, but I still have you beat on Wisdom, Dexterity, Strength, and Charisma, so don't get overconfident."