Ugh, I feel you. My cousin used to hit me up for free computer fixes all the time. I finally told him straight up that my time has value, and offered to do it for a discount instead of free. It was awkward at first, but now he respects my work and only asks when it's serious.
When he says "it's just a favor," that actually downplays your skill and time. While @paulsanchez has a point about charging, consider that always doing it for free might make him see you as a fixer, not a friend.