If you don't plan to seek funding it doesn't matter what you do. If you do plan to seek funding, figure out a way to make it a 50/50 split if you are going to have a cofounder, any other arrangement will raise eyebrows and cost you time and legal fees when you try to get funding.
I earned twice as much as him when I was working and quit my job and didn't earn a salary for 3 months. These 3 months alone are worth more than he is investing. The sum we both invest should be sufficent to get the company running for at least 10 months without any revenues. If it turns out to be a viable idea, we both (or at least I) can invest some more money or seek for an external investor.
I don't think a 50/50 split would be fair, as I already proved that I could do my technical park, and he only joined afterwards and has not yet shown or done his part.
If everything fails, there is still the option of selling the software. At least my former company will be very interested in what I was doing.
I don't disagree with what's fair, just saying that if you have a non 50/50 arrangement then you are probably going to have a different sense of when it's time to cash out than he is.
Why not just make him your first employee and compensate him with incentive stock options? He would not be a partner or a founder, but a fairly large potential shareholder if things go well.
