1) You need to enter into a contract agreement with the developer, there you must mention that, you have all copy rights of the mobile app.
2) Without Source code is a wrong decision, you have to be always dependent on customization or further development with the same developer. Please try to take source code along with application.
Ask if you are not clear
You can apply for copyright registration in respect of your software/app with the Registrar of Copytights. You can apply for the copyright even before publication of the app. And you can also entre into an agreement with the developer to protect your interest.
You can consult with us for further proceedings: firstname.lastname@example.org