I invite you to look at some video clips on my website of very young kids hitting beautiful strokes.They all are being taught with Oscar's Modern Tennis Methodology. I also have taught my grandson, now age 6 1/2 and he and I have sustained rallys from all parts of the court, plus serve the ball in. In the beginning he didn't want to take instruction from me, but over time, as he began to undersand more and watch me teach other children I gained his confidence and respect. My ability to get quick results with players of all ages and ability levels has increased as I become more familiar and comfortable with Oscar's method. Please go to www.tennisteacher.org and click on the Video Clips page to see how magically kids respond to this style of teaching.
There are many facets you'd need to look at, but first of all, If your son is just getting started, I think the best thing to do to keep him in it is to play some mini-court points and do not beat him too often! If he feels he can rally with you and win points, tennis will get to him. Feeding balls and let him hit may be fun at the beginning, but in a while, he'll be bored - that is common among many kids of that age. So mix it up.
I made a mistake with my son when he was little. He could play mini-court point with me and that was how he got in it. But I asked a little too much, and too soon, from him, by feeding balls to him and kept telling him how he should do fh and bh, and then tennis became boring to him. So now, whenever I ask him, he'll find every possible excuse not to play, and when he runs out of excuses, he'll just say, let's just do it for fun, meaning, just play the points (and let him win).
So, this experience might be something you want to consider.