tl;dr: Some guy I just met says he wants me to join his company. Before he shows me what they do, he wants me to sign a non-disclosure agreement. Is this weird?

I'm traveling right now. Someone who saw me coding seemed to think I was smart or something and started talking with me. He explained that he owns a software company, told me a bit about what it does and told me that he was looking for a programmer who would work for a stake in the company. He explained that the company's product is being developed rather secretly, so he couldn't tell me much. But he did tell enough about the product to convince me that he's not completely making this up, which is a decent baseline.

He suggested that he show me more of what he's been working on and, after seeing that, I decide whether I want to join. Because of the secrecy behind the product, he wants me to sign a non-disclosure agreement before we talk.

I'm obviously somewhat skeptical because of the random nature by which we met.

In the short term, I'm wondering if I should be wary of signing such an agreement. He said it would be easier to show me the product in person rather than over the internet, and I'm leaving town tomorrow, so I'd have to figure this out by tomorrow. If I decide to talk with him, I could decide later whether I trust that it's worth spending any time on this company.

The concept of being able to avoid telling a secret seems strange to me for the same reason that things like certain aspects of copyright seem strange. Should I be wary of signing a non-disclosure agreement? Is this common practice?

I don't know the details of the agreement of which he was thinking, (If I end up meeting with him, I'll of course read over the agreement before I decide whether to sign it.) so I could consider alternatives according to the aspects of the agreement. Or I could just consider the case of an especially harsh agreement.

This question seems vaguely related. Do we need a non-disclosure agreement (NDA)?

Thanks