Did you have any recent lab blood test results to discuss? If not, I would ask for some as soon as possible. It is likely they will show your diabetes to be improving. They could also act as a lever to open his mind a little to the alternatives to his mantra.
My surgery are wedded to the usual NICE mantra, but in discussions a few months ago about another issue I had at the time (completely non-diabetes related), the Advanced Nurse Practitioner was utterly fascinated by what I had achieved with my diabetes and how I had done it. Coincidentally, the Rangan Chattergee series was also airing just then, and we had quite a long discussion about sausages (I know, unbelievable really). She just kept saying "I thought I had a decent handle on food, but I need to do my homework".
As you can imagine, I left her a little reading list. I like to be helpful, where I can.
If only the senior partner, who is the practise diabetes specialist, apparently, had a more open mind...........
(He's my work in progress.

)