It didn't come across to me as creepy. My guess is the father passed on to his friend looking for advice about his son. The father may have even mentioned to his friend my son is bigger than me and perhaps confided in the OP knowing he is endowed. You people sometimes jump too quickly for innocent questions. You don't have to be a pedophile at all to notice a kid with too tight pants on and a large endowment. It isn't that you are looking, it's just kind of obvious.
The fact that he mentioned his son wearing a jockstrap I thought was a good one and also less snug pants.
The only part of the OP's ? was that the kid was naive. That doesn't mean dumb, that just means to me not too street smart (which would not be an insult).
I went back and reread the original post. It still doesn't seem creepy but this part should have been left out...
"(like I said the kid isn't the brightest bulb in the chandelier)." Beyond that statement, it could be an innocent query, especially to guys out here that are well endowed with memories on how they handled it.
Lighten up......