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.
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solved How can I make sure my app doesn’t get stolen by a developer [closed]