A 64-year-old man completed radiotherapy (60 Gy) for a tonsillar cancer 4 months ago. His mouth is very dry, he sips water constantly, and at review you find early demineralisation at the cervical margins of several lower incisors and premolars. He brushes twice daily with a 1450 ppm fluoride toothpaste. What is the most appropriate long-term preventive measure?