I would think it is time for a come to Jesus conversation, gauge level of interest and if they aren't on the same page as you, it's time to cut loose. If you let this fester, it's going to result in resentment and it will eventually unwind, do it now (if needed) in a congenial way vs. combative.
It sounds as though they need to reconnect with their and the company's values. I would sit them down and discuss why you set your business up together and what you hoped to achieve, then look at and really appreciate how far you've both come since then. I would then ask them, if you're sitting there together 3 years from now, what do they want to have achieved/have happened in the business for them to be happy? You may need to create or modify a strategy to provide meaning to that vision, but align it with company goals and your goals too, and hopefully they will have a renewed sense of purpose.
This article is really good: blog.strategiccoach.com/magic-vision/
I agree with the others.
I'd probably take a conversation with the co-founder to find out whether he or she really wants it as bad as you do. I believe in the ability to motivate, but you should motivate your employees, not the head of the company; and even not the co-founder. If he does not want it bad, he should not be on it.