Well, my first thought (honestly, go ahead and flame me, y'all), was:
Barf.
Because as an educator of young children, that is the last thing they need to hear....way to set them up for failure, truly. I hope your son hasn't heard it. And if someone said it about my child, I would definitely acknowledge them, but also say, time will tell, right now he is 5....and my main goal is for him to have fun while he learns this new sport. He is a mere 5 years old. As I don't want animosity from other parents or gymmates, or a child (yours) who could get hubris very easily and then think he therefore doesn't need to try, or get frustrated to the point of feeling like a failure if a skill becomes challenging, because, being a prodigy, all should come easy. If my child HAD heard it, I would make sure to explain all this to her also. This sport has lots of haters, sadly. And lots of jealous kids and parents. Luckily with boys, it doesn't seem as bad with the 'boys...'.....but parents still are and can be pretty darn intense.
Obviously you have overheard this......what have you said? Not being rude here, but it sounds like you have already had this happen more than once. When you don't nip things in the bud, they continue to grow.
I do get a coach's excitement if they have an athlete that looks promising, and that's great. I also get how excited you are about him being on a team. But I would try and tone everyone down a little. If I recall you have been wanting him on a team for quite some time....(if I am wrong on this one, I apologize)....I think at age four you put him with older boys because you felt he needed more....
Said kindly.....
try to keep it fun. And keep it light.
My two cents.