What is your suggestion for demonstration on his part? Signed statements prior to sex? Video footage of discussions beforehand?
Yes, how could an adult possibly know they were dealing with a child?
First of all, men have to start accepting that sex isn't always on the cards. If she's too young, it's no. If she isn't too keen, it's a no, even if she isn't trying to kill you to get away. There are too many excuses built into the social discourse around sex for men to evade normal social responsibility if sex is on the table, far too many excuses, far too many people ready to take pretty implausible explanations as fact.
I actually think that people going on about video evidence and that kind of crap are part of the problem of endorsing this idea that establishing that a prospective sexual partner is willing and of age as requiring mind reading superpowers rather than the minimum necessary as a responsible and normal person is part of the problem leading to men not being seen to be responsible for their actions.