This all depends on the contract you have made with him. You should have set out at the beginning what deliverables he should provide.

1) No he cannot change your app that is currently in the app store.

2) Did you agree at the start that he would provide you with the source code? Or did you just agree that he would deliver the binaries to the correct app stores? If you didn't agree for him to send you the source code he may ask you for more money to provide the source. This may sound like a dick move to some people but this is generally the way things work.

3) If/When he does send you the source code then you either need to trust him or get another developer to verify it for you. You could;d just ask him to provide build instructions with the source code. This would allow you to just build the project and run it in an emulator on your desktop. You don't really need to be a programmer to do this and you should be able to figure it out with some instructions.