How can I get my son to use seat belts in the car?

Persistence . Children often will challenge family rules and guidelines. Simply not driving the car while he is unbuckled or after he undoes his buckle/car seat can be very effective. Some children are particularly adept at 'escaping' from car seats/seat belts. Simply pull over, calmly ask the child to get back in, and check to ensure everything is snug. Staying calm, but firm takes the 'sport' out of it.
Seatbelt. You need to enforce the use of seat belts explaining to him how dangerous it is not to be wearing one. This may require strict guidelines either incentive based or punishment based. It is extremely important to get him in the habit of using one.