With a 6 years old, it's tough 'cause you can't reason them out of a bad habit... The best thing is yet to have them play a game whose rules force the good habit in place.
Use the net and have him roll the ball over it a few times while explaining him that the next lesson he will learn will involve spinning the ball. Don't get too technical as he won't understand any way; simply make it fun. Then, propose him to simply flip the ball with spin over the net: the goal is to spin the ball into the other service boxes. At first, hand feed him -- just make sure that he's forced to lift the ball over the net and that he hits appropriate spin. Once he gets good, ask him to do the same thing, but this time he has to aim deeper.
A big mistake people might make is tell their kids things that follow the form "if A, then B"... that's a deduction and they cannot yet do this mentally. You could go and tell your kid that if he hits stop spin, because it brings the ball down earlier, he will then lessen the number of his mistakes, but he cannot operate these things -- he doesn't think that way, yet. At 6, it is plausible that he might be able to generalize stuff: if he's there cognitively, he will understand that when he does as you asked, the ball drops earlier and will use it when required. But, even then, he might still not be able to do this sort of operation before a while... that's what I meant by "you can't reason them out of a bad habit."
With a kid of his age, you need games, funny things... anything, in fact, that have them take good habits without it being in anyway formal. He's probably just hitting big and flat because he likes the feeling of it, but even a baby can reason this way. If you want to have him understand what he's doing, a lot of mental imagery, something symbolic, like having the ball draw a rainbow, can be much more effective as he should be technically thinking with symbols at the moment, unless he's early in his development.