Ask HN: What deal makes sense for a late co-founder?

I'm about to join a startup that has existed for a couple of months on paper, but hasn't actually done anything yet. It's the infamous example of an "ideas guy" looking for a programmer (me) to build it, but he's already been working hard on collecting information we need, networking, talking to potential stakeholders and trying (unsuccessfully, so far) to get money for this idea.

I think the idea is great and would be fun and educational to work on. The only downside is that he doesn't have money to hire me.

We're not sure how to arrange ownership of the startup, particularly in light of the work he has put into it. The first heal he proposed was: I get a normal salary, and in 3 years I can buy 20% of the company for €2000. As long as he can afford to pay that salary, that's a totally fine, no-risk-for-me deal. But there's no money, and I think maybe it'd help a lot if I built a prototype first. I can afford to go without income for a while, but since I'm unpaid, that requires a different deal. Some of his proposals:

* If I work 1 month unpaid, I get 20% of the company.

* If I work 3 months unpaid, I get 40% of the company.

Both seem fairly reasonable to me considering he's already put several months into this.

Something I'm considering is: the prototype is mine until we get funding, and then he buys it from me. This would have the advantage that if it fails, I keep my own code. (But is that worth anything? Once it's finished, I can probably build a better one in less time.) And I'd get money for my work instead of shares that may or may not be worth anything.

Downside is that we don't know when we will have funding. But that's really the fundamental problem here: will we have some funding in 3 months? And how will we eat if we don't? What risks are reasonable to take here?