I do have this problem (just talked to the dentist about it at his appointment this week). She suggested an adult tartar control toothpaste, but, like your son, my son doesn't like them. We bought a Crest Complete for Kids - it doesn't specifically say tartar control, but the adult crest complete does help tartar, so I'm hoping.
We also went the other route someone suggested, and got an sonicator toothbrush and I'm trying to make sure he does a thorough job brushing. I don't know if it will work or not. I guess I'll let you know when he goes back in 6 months.
Also, FWIW, my dentist told me not to feel bad about it. My son has allergies and she said that is a huge culprit for tartar. If you don't treat the allergies, then the front teeth get dried out from mouth breathing and tartar accumulates. If you do treat the allergies, most of the medications are drying to the mouth and tartar accumulates - so you get tartar either way.
UPDATE: I just looked at home, and it's actually Crest ProHealth for Me, not Crest complete.