The kid is ten years old so this conversation seems more than a bit creepy - but the Constitution doesn’t bar him from being elected President. He’s a citizen by birth and his mother’s citizenship is irrelevant.
The question was even part of the citizenship test, when Melania took it; so there!
Well, if we were to go by the strict originalist interpretation of citizenship re non citizen mother - Barron would qualify under the citizenship laws at the time the Constitution was signed.
When a US citizen male, living in the US, married a non-citizen, she became an automatic US citizen, and all of their issue were certainly as well.