I think people are too quick to say "fire the cofounder" in these situations. It's very easy for software development to take months longer than expected, even when you're about 95% done. Finishing stuff and having some judgement about when a product is ready is really, really hard.
The issue seems to be that he's telling you "just need to finish one more thing" every time. Get a list of exactly what needs to be done in this kind of format with estimated amount of time...
* Task a - estimate 5 hours
* Task b - estimate 3 hours
* Task c - estimate 15 hours
* Task d - estimate 10 hours
* Task e - estimate 5 hours
The issue seems to be that he's telling you "just need to finish one more thing" every time. Get a list of exactly what needs to be done in this kind of format with estimated amount of time...
* Task a - estimate 5 hours * Task b - estimate 3 hours * Task c - estimate 15 hours * Task d - estimate 10 hours * Task e - estimate 5 hours
...and work methodically through the list.