First make sure if he is doing it on purpose or if he is really bad at making calls. Most cheaters make bad calls not (only) to get a free point, but more importantly, to get in your head. They watch you getting more and more worked up after every bad call and missing more balls.
I play mostly on clay fortunately, so i can make them point out a mark if i question a (not the first or second) bad call. If they are obviously cheating and will not stop, i will shake their hand and walk off court if it is a friendly match. If it is a serious match or competition, i will not give them the match that easily. I will keep questioning their calls and if it doesn't stop, i will make one or two bad calls myself on a crucial point. But then i mean a really bad call, preferably a ball that lands within 2 feet of the lines. Then watch their reaction as i state it is my side to call and i saw the ball out.
Either way, i will never play these opponents for the second time.....