Non-technical founder asking for a larger stake in the company

Possible Duplicate: Forming a new software startup, how do I allocate ownership fairly?

I'm in a position where a colleague has offered me a percentage of a startup that is just getting off the ground, but I'm a little unsure how fair the offer is that he is giving me. I really do believe in the product that we'd be making but I'm not confident in this person as a developer (though he has a lot of value in other ways ie. running the business / clients / etc.). The value that I would bring is that I actually do have the experience and capability on a technical level to actually create this thing.

He has already quit his job to devote himself full-time to the startup, but I'm not able to support myself financially to follow suit just yet. I am taking a month off work starting in a few weeks (the same time he is going full-time) to throw myself completely at making the prototype (after which I plan to bootstrap until we get funding). He is suggesting that he retain 55%, while offering me 25% and another developer 20%. I also have very little confidence in the abilities of the other developer (currently offered 20%) especially given that he is also planning to remain part-time.

Basically I feel like I will not be compensated fairly for doing all the actual work in creating the product (even though yes he is taking a much larger initial risk by going full-time). Negotiations are still on the table but this is new ground for me so I'm unsure of how to proceed.