Diagnosis refusal

Hi Each, I could really do with some wise words right now. After repeatedly asking CAMHS to assess my now 15yr old son over a period of 7 yrs. We finally got the diagnosis that I knew was inevitable.. The problem is is that my son is declining to accept the diagnosis after being told by one of the assessors that he could choose not to accept a formal diagnosis. Whilst I fully understand that he is entitled to make this decision I am absolutely gutted by this. He's due to start college in September and is going to need a lot of support to make the transition. It would be so much easier to be able to say that his challenges come from Autism not bad behaviour which was how he was treated at school. I cannot bear the thought of having to endlessly advocate for him whilst withholding the single piece of information that would unlock the help he so desperately needs because he is anxious nearly all of the time. I know I have to respect his decision but i feel so let down that after all the worrying and fighting for his rights we finally have an answer but cannot use it. I know that I'm probably being selfish and am so deflated by this turn of events. We waited 2 years to even get the assessments done. Any wise words would be gratefully recieved