Is he not motivated to work on his footwork/ foot speed and endurance because he'd rather do something else ( eg. playing video games , etc)?
If that's the case, I would stress on conditioning and benefits of exercise to him.
Playing college tennis or not, any kids should exercise 3 times a week. Give him this health reason to motivate him. Good conditioning can improve outcome of his matches.
If he got discouraged from the outcome of his high school matches last year, I would not use reward for good results but emphasize on other things instead. Results will come if he improves in other areas needed improving and less worried about results.