How to make a paper's author willing to help me with his source code like a semi-supervisor?
No one can really 'make' anyone willing to help you.  However, some things to consider that may convince the author to help.  In your email communications:


demonstrate your understanding of the source code. 
Be specific in what you ask - don't do the "I don't get any of this" type of message.  Use your understanding, research and testing to help focus your questions.


Make sure you have your understandings and questions ready before you email, do not send multiple emails before they have a chance to reply and try and put most, if not all of your questions in a single email.  Be patient when waiting for a reply and be courteous and thankful for information provided.  

Have your supervisor/advisor look over your questions before sending it (if you're unsure).

Remember, most of all, the author is under no obligation to even reply, and definitely not under any obligation to do your work for you. Bear in mind that they will have time-taking obligations and duties that take priority.