Take him as a co-founder - no doubt about it. Give him a 50/50 stake with vesting. It is best if you also clear leadership issues and decide on a CEO. The ques

Take him as a co-founder - no doubt about it. Give him a 50/50 stake with vesting. It is best if you also clear leadership issues and decide on a CEO. The question that you should really be worried about is how to make it work (read manage conflict) between the two of you. Consider him your startups greatest opportunity as well as the biggest challenge. Apart from the reasons others have already mentioned for having a co-founder - You need a co-founder not just to save your startup but more importantly and more frequently to save you from your startup.