Has your co-founder brought in any sales? Does he have any industry expertise? A strong network of customers/advisors/investors? Is he leaving a high paying job? If the answer to all of the above is no, then it's hard to justify giving him much...
It sounds like you've done all the heavy lifting, but understand there's a lot more to come; ask yourself whether you want to do this on your own; ask yourself whether you want to do this with anyone else.
No one wants to leave their job for anything less than 10% (with no salary). Also, if he's 'your guy,' make sure he feels like a co-founder; someone who can share the pain.
Finally, make sure you sign an agreement wherein the shareholders earn equity after a specific date (this is called a cliff, it typically occurs after 12 months), and that equity vests over a specific period (typically 48 months). You can find a lot of information on shareholder agreements online, but at some point you'll want to see a lawyer.