This is one of the toughest situations to be in.
First, figure out if it’s just anxiety. Starting something is hard. Getting to the first $1m in ARR is borderline impossible. If it’s “just” anxiety, and your co-founder is truly great at something — see if you can help him stay the course. At least for now.
But if he’s checked out — if he’s going to leave. 100% for sure. If he just can, or doesn’t, want to do it anymore. Then, just let him go.
This may be the hardest thing you’ll ever have to do, to let a co-founder who is truly great at things, even a co-founder you simply can’t do it without … go.
It may seem impossible without him. It may well be.
But dealing with that sooner rather than later increases your odds of success.
Great founders never quit.
Sometimes, though, they need a pep talk or two.