I was in a somewhat similar situation and was fretting about it for weeks. I had founded the project and done all the work, things were going well, and I asked a friend to join as co-founder. I felt after a few months that he wasn't pulling his weight, and I was concerned about how to approach this, and how I would ask my best friend to leave the business.

Turns out, he had already been interviewing and wasn't keen on staying. Start-ups weren't his thing, he wanted more stability.

The best thing you can do. Talk. If he's going to be your co-founder, you want to be able to have frank conversations.