Dealing with a friend who always leans on me

I major in Mathematics and Computer Science. I have a friend who excessively depends on me when it comes to coursework . He wants me to explain definitions, proofs, share code, check his proofs and bugs in programs. It seems to me he does not pull enough efforts when he doesn't get things in the first try and approaches for help.

I have tried to advice by saying "you should work on this for more time" as most of his questions are trivial. I think that he finds it easier to get help from somebody rather than reading again or spending more time. I also think that I am spoiling him by helping. This is also ruining me as this eats up my time.

What is a good solution to my problem? How can I convince him to put more effort from his part? I tried to avoid him especially when exams get near, but he ends up finding me.