A.
You didn't mention if your son has gone to the store with you and picked out his own underwear. Sometimes picking his, putting on his own and keeping them clean is that last bit of incentive a child needs. We also used quick, simple positive reinforcement (sticker chart with little stickers for wearing the underwear, big ones for keeping them clean and sitting on the potty, and huge ones for potty success - racetrack/train track charts are big hits in my house - when the track is at the end, the child gets to pick (restaurant, toy, book, etc)., Short tracks to start for quick success helps too.
As for the seatbelt, we had a strict rule on that. There are just some safety issues that are not up for negotiations. We have more luck with positive reinforcement than negative, but each child and situation is different. Regardless of which you choose - consistency is key. Good luck!