This is strictly an opinion of course, but it seems like your co-founder is afraid to launch (and everything that can go with it, like failure). Sometimes it's confused for perfectionism, but you can tell the difference by the lack of progress / improvement. Fear is one of the most common impediments to success, and people like to talk about it approximately as much as they might erectile dysfunction. Once you launch a project, your life changes; the pre-launch development stage is substantially different from the post-launch stage; it places demands on your shoulders that you can't shirk off (properly), it dictates a lot about your life. And there's no going back, it's sink or swim.
Talk to him about the situation openly, lay your concerns on the table. Try to get a drop dead date agreement, after which you take over the project.
Talk to him about the situation openly, lay your concerns on the table. Try to get a drop dead date agreement, after which you take over the project.