I am a dentist in the uk (not an orthodontist however we are an orthodontic practice so have them working closely with us).
Firstly, I’m not sure what he was saying with regards to reporting you. A social services report wouldn't go very far in this case! If he was saying he would report you to the PM, perhaps he felt like you were being aggressive. I’m not saying you were but often patients (or family members) frustration, can come across as aggression, and every practice I know has a no tolerance policy when it comes to this.
I do know that it is policy for an orthodontist to ask every patient if they want braces and for a patient to decline, is an automatic no. I understand in your case there are other things to consider which is perhaps why he’s saying come back later, rather than just rejecting the referral. Braces cost the NHS approx £3000 which is why this rule is now in place. It’s a relatively new thing.
With regards to gillick competency, I agree that he your son most likely isn’t. However we still cannot force teens into doing treatment they don’t want. I appreciate, however, you saying it’s perhaps the language the orthodontist used. And in this case, I agree he could have changed his language or approach. I have seen many SEN patients and always take into account their literal thinking.
On the other side of the argument, for someone with SEN, braces can be really difficult. They’re painful, they irritate the mouth and tongue, they’re different and new, and many can’t tolerate them.
I don’t agree with how your interaction went, he should not be saying he will report you for being a concerned mother. And as I mentioned already, it would not go anywhere if he tried. So please be reassured in that respect. I just wanted to put across the perspective as a dental professional. Please feel free to DM me should you have any further questions