I haven't started negotiation with him yet
Yes, you have. You made an offer, he rejected your offer. To get a different outcome, you need to make a better offer or assume this was a bluff and be patient. I don't think either of these are likely outcomes in your favor.
I think the language you've started with: convince, negotiation, suggests you do not understand your position. Offering to work for funds is not a big offer: it's a neutral trade at best if you're a good candidate and a position is open. I would think less about this as a negotiation and more about what it is that makes you an optimal candidate, better than any other candidates, that would justify opening a position that may not otherwise even exist. Probably a better approach for you is to aim for many possible opportunities with hope that you will be the best candidate for at least one of them.
Good luck!