(1) Will he be able to make any changes to the app which is UP and running on the market place without logging into my developer account? I don't think so, but want to reconfirm.
(2) It looks like it is better to have the source code in case if i want to make any changes in the future. In this case, what kind of format i can ask him to send me the source code?
(3) I am not a programmer and don't know ABCD of game programming. So even if sends some thing, i cant really make sure that this the source code for this program. What are my options in this case except trusting him thinking that he is sending the right code?
2. There is not really a format. As long as you get the code, you should be fine. If he has some fancy project setup, then it would be great if he sent you those or told you how it was set up.
3. I doubt someone would do that. Especially if you have made a game with him. Anyway, I would open the source files and see if you can find things you recognise.
For example, see if you find the code for the player or the code for the main menu or enemies. Just read the code a bit like English, and you should figure out what it does.
If you have no idea after that, you should perhaps either trust someone else to look over it, or learn to build the project yourself given you have the project files.
Got ninja'd. Lactose probably has a better answer than mine though.