Have you had a conversation with him about it? The co-founder relationship's not the same as an employee/er relationship. Maybe he feels like you're bossing him around and he's unhappy about that. Maybe something is going on in his private life that's distracting. Maybe he's just bored - after all, finishing a project is paradoxically more difficult from starting one.

In the absence of information from him, do you think that it's worth finishing the product? You could almost certainly find a freelancer to finish it, but that's going to take cash that could be invested elsewhere.

I'd talk with him before contacting a freelancer, though. Unless you're planning on butting him out completely, he could see this as you going behind his back.