IANAL, etc. But I'll give you a plan anyway. And you should have a lawyer help you with it. The plan may be overkill, you may be able to skip directly to the "real action", below, but following the plan makes sure you have your ducks in a row before taking "the action".

Create a corporation this week, yourself, ideally with one shareholder, you. If you need multiple directors, have the other director be someone you trust (best friend, brother, mother, etc.), anyone other than "the co-founder" (whom I will refer to as "the other guy" for the rest of this post.)

(You may be able to immediately pass a directors' resolution or shareholders' resolution reducing the number of directors, depending on your jurisdiction.)

Assign all copyright in your work to the corporation (possibly in exchange for a nominal fee, e.g., $1). At this point, the company you own controls the software. If necessary, and upon advice of the lawyer you hired, file in the corporate minute book copies of all emails or other communications between you and "the other guy" that show, explicitly or implicitly, that you wrote the code and were simply making it available to him (for purposes specified or unspecified). At this point you have documents that may be considered evidence of your effort and ownership and of his non-involvement in the production of the code.

(Be sure to include in the above batch of documents anything that does reflect his contributions, e.g., in terms of reviews, recommendations, suggestions.)

The real action step: Once the above is done, send him a brief email indicating that you aren't sure whether or not you want to continue with the project (call it that, not business or product or company or anything else) and that you are willing to sell him the intellectual property you have developed so that he can continue with the project. You do not have to say why you are no longer interested. If pressed, cite "work life balance" or something.

What happens next depends on his tone. If it is anything other than friendly and understanding, indicate that you are disappointed, hope to be able to resolve things amicably one way or another, etc., but don't make any commitments or say anything other than pleasantries. If his tone is still other than friendly, play hardball. Have the lawyer or a good friend do it if you are uncomfortable. Tell him that you are going to sell your interest to the highest bidder and are offering him first dibs.

If his tone is friendly, tell him you want to work out together what happens next, that you incorporated, for your own protection, and either that the two of you will agree to his share of the company (which is entirely in your control) and agree to work together or that you will sell him the company and all interest in the IP for a reasonable and mutually acceptable price.

If you agree to work together, you may want to write down roles and responsibilities for each of you and documented expectations for the next several weeks/months.

If it looks like things are going to go south, and that a sale is a pipe dream, and that you are not interested in pursuing the project, then, as long as you know for sure and for certain that you own everything, open source it with a license that prevents others from "taking it proprietary" and building the business in your absence without rewarding you.