Started this project together, split the equity equally.
Idea was my co-founders', development work is mine. We still work great together, but he just can't put enough time on the project due to family, other work commitments etc., while my engagement continues to rise.
It's only been 3 months, but I would have worked 3x more and I'm full time on this now. I can clearly see that in future I would probably be handling 90% of the company work while he pitches in <10% of the time, as we keep on adding new features. The problem aggravates if we grow in numbers: it won't be fair to new people that someone who is working much lesser on the project has all the equity.
So this seems like a situation many would have been in. How do you suggest I handle this?
Having cofounders is a touchy subject and any problems arising between you two now is pretty much a guarantee that it will destroy the company in the future. If everything has already been signed and there are no vesting agreements in place, the only way to do this is to buy out your co-founder to avoid any future legal issues. Or you could dilute your co-founder as you bring on outside investment (e.g. Facebook). Either way, there will be some emotion involved. I would shoot for buying out your co-founder in some way, perhaps with a loan of some sort. It's the cleanest and most quickfire solution, especially with you so invested in it. Of course, this is highly dependent on the assumption that you are passionate about what the company is doing and that there is still a great opportunity here.
Props to you though on choosing to deal with the situation now rather than later. Good luck!