Not money rewards but things to do at your house when he comes. If he passes all his classes he can xxx (go somewhere with you, get something while he is at your house, have one on one time with you doing something fun/cheap- and you can do the same thing for your other kids).
Next time he is with you tell him how your feel about school and graduating. Leads to better jobs if he finishes. Show him some facts from the internet about salaries. That you will help in anyway you can and that you know mom may not have the same views but this is important. Then come up with a few ideas to encourage him (can make it a family meeting and have all the kids pick their rewards). If you see him after the first quarter, then when you get those grades, when he comes for thanksgiving/christmas....xyz. Then spring break/Easter, Then end of year- summer. However it works for you.
He can do chores at your house and get a bank account at your house with your signature so he can start saving for a car/truck/college whatever... Even as a NCP you can do a lot of things but the ex will try and get in the way. It sounds like he is older so hopefully he will see what mom is trying to do and see you are trying to get him to graduate.