What if you were tell him the same thing?

Why on earth is he to trust you, but not vice versa?

You two are now equals and peers and it's time for you to start acting like it. If that's unacceptable to you then you never should have put yourself into this situation - a cofounder is not an employee.

Such things do happen, but rarely. The only way to really "protect" yourself is to be the most useful possible co-founder yourself.

In any case, having access to your source code will not be an advantage in competing. Building new products is hard because you are at the time also figuring out and changing what to build.

If your idea is any good and has proven moderately successful, there will be competitors. Competitors who are not your co-founder and have never spoken to either of you, but just see your product and decide that it would be cool to have it in their city, country or business niche. And they will spend only 1/10 of your efforts on development and end up with a more maintainable codebase, because they have an example of what to build right from the start.

And what if he decides to keep going, to realize a few weeks later that you reused the very same source code to help building a company's friend? I mean, everything goes both ways. Don't expect to be trusted if you can't trust

This is why you have contracts and lawyers.