Refrain, ask him what he will do after he has finished his pro career?
Remind him how embaressing it would be to have to go BACK to school when he is older, versus finishing it now.
Tell him that education is something that will enhance his entire life, and will be worth it hundred-fold when his tennis days are done and gone.
Generally the people who have truly long, great, or powerful careers (in anything) have been educated, or seek education once they can.
Ha, tell him that it is a no brainer question, and if he chooses to give up a good education he obviously has no brain. Ha, don't do that, but I thought it was funny. (This is what happens to my sense of humor when I am writing a history paper...downhill downhill downhill)
"If you can talk, you can sing...if you can walk, you can dance."
- T.K. Greene