[dirktheman]

So you found someone who's willing to do a lot of work with you? Without a guaranteed pay? And he has invested a lot of time learning backend stuff so he can actually help you building and scaling your business? Awesome!

A co-founder is not just some random person with complementing skills. It's more like a marriage. And while marriages often start out good and, yes, can turn bad, it's quite unusual to not trust your newlywed wife with the keys to the cupboard.

I'm not trying to be negative here, but it seems that you're afraid your co-founder is learning too much about backend development, so he can steal your codebase and his front-end design, and steal your entire business and become an instant millionaire. Or he could actually be genuine, and willing to take the risk and reward together with you. Out of these two, which option seems more likely to you?

You say it yourself: you have no customers yet. You can't just steal a codebase and copy/paste an entire business to success. Per your words: 'once I get customers'. You don't have customers yet, ergo you have nothing. Stealing your codebase wouldn't make any difference.

You have to ask yourself if this person is really the person you want to start a company with, and work closely with for years to come. If he is: give him access. If he isn't: do both of you a favor and part ways. Nothing good will come out of this if you make him earn your trust whilst being co-founder at the same time.