If you give him your source code, no technical measures can stop him from stealing the source code. If he were local to you, you could require him to use a locked-down computer in your own office, but I don’t think that is an option for you.
If your app has substantial non-code assets (graphics, sound, databases, etc.) you could give him only the minimal subset of assets necessary for testing.
If the app talks to a server you control, and you don’t also give him the code/data to set up his own server, then him stealing your code may be less harmful.
Other than that, you need to consider legal enforcement, not technical enforcement.
solved How can I make sure my app doesn’t get stolen by a developer [closed]