[trjordan]

There is probably something else here that you need to talk to him about.

Why does he feel the need to code up a feature right now, without talking to you about it?

Why is he having trouble keeping everything about the product in his head, with looking in the code as the only fallback?

Why sort of gaps are there in the demo that looking at the code is the best way to address it?

This is not a problem with him, though it's also not a problem with you. It's a problem with the business where the founder who talks to customers feels he has to come back and bang out features. Worst case, he's just bad at his job, but the likely reason for this is that there's a mismatch between you two in terms of stability, urgency, or prioritization.

You're adults. Talk it out.