You are effectively at square one if this becomes successful. In the end it's all about agreement between you three. In my opinion if this is a person you value

You are effectively at square one if this becomes successful. In the end it's all about agreement between you three. In my opinion if this is a person you value enough to join you as a cofounder at this early of a stage, and he will be putting in the same amount of work as you and your other cofounder, I would suggest making him an equal or close to equal partner. If you don't see him as valuable as your other cofounder, you can always offer a significant equity package as a first employee. I know the difference may just seem like a title at this stage of the company, but it makes a difference in terms of what kind of information and decisions he will partake in and the magnitude of volatility in the emotional startup-wave he will be riding is.