I think that its concerning that you've spent this much effort into thinking about shares. The fact that you're second-guessing his contribution or value means

I think that its concerning that you've spent this much effort into thinking about shares. The fact that you're second-guessing his contribution or value means something very bad will happen between the two of you. It also means you think you don't need him - if so, then don't partner up. If you truly wanted him as a co-founder, how could you dismiss him over 10-15% of a currently valueless startup? Cofounding a team shouldn't be a dissection of options. It should be obvious that you haven't found the right person.