a) that's a pretty large time investment
b) $2500/mo for 80 hours of consulting is pretty poor pay -- $30 an hour?! That's $60k/yr with no benefits...
Seems like a pretty poor deal to me.
Some people will tell you they did this and it worked out fine, others will tell you it was the cause of their demise.
I don't mean to attack your question, it just seems that you're asking for an answer to a multi-variable problem, and you've only given 1 constant.
Also, what would happen to that $2500? Is it to pay his bills, or fund your venture?
As for our roles in the startup, he is a web developer, I'm a mathematician/developer, so we complement each other. Just the two of us, at least for the coming couple of months. The idea that I'm getting, already in this phase of preparing for the startup, is this: in a startup, there's always work to be done and you get bonus points if you iterate fast. So that kind of mitigates the factor if somebody is a good multitasker or not (I'm more of a believer in focus).
it's going to be one of those things where you have to decide what's best on your own, because you are the one with insight as to what needs to happen.