Could you encourage your son to go to a young adults clinic? There are various hospitals which run transition clinics some up to about 21 years of age where they are very good at encouraging people through this difficult time whilst getting a grip on controlling their diabetes. You are not treated like a child and maybe this approach would help him.
Is his lack of trust of the nurse and diabetes GP purely down to his fear of blood tests? I went through a period of refusing to attend clinics and put the reason down to hating blood tests. The reality was I hated the thought of the results but blaming the needles seemed a believable reason. I was well aware at 18 that I wasn't managing my diabetes and I was also well aware that I was now classed as an adult and nobody could make me do anything.
It is a really difficult time moving into the adult world but there is some great help out there. I would try to find out what clinics are available to him and encourage him to think about seeing someone different to see if a new approach will help. I wish you good luck.