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."

Awesome. I also like "In what way?".
My response to DS was the observation that a true appreciation of one's abilities is a strength, but that a big, brittle ego is a tremendous weakness (we had previously discussed Carol Dweck's mindset research). I also told him that my goal is to help him reach his full potential, and that I will be glad if he does so, but that it will take hard work. I remember also saying that no matter how capable he gets, I'll always be able to beat him at something, even if that's just pooping in my diaper.