Thank you everyone for your replies and to the AMAZING person who has emailed me.
Today I called my local CAMHS and spoke to triage, who suggested going BACK to my GP and getting a pediatrician referral. The pediatrician will then do some assessments and the triage person said he's pretty confident that DS would be referred back to them, but without those assessments I'd be run around in circles for a while, so even though it feels like a huge step backwards (going back to the GP), it will make things move faster ultimately. The triage bloke also mentioned the spectrum several times even though I'd only mentioned 2 of his quirks, so I'm confident that there will be some form of diagnosis that we can work with.

I also spoke to a child Psych who is calling my tomorrow afternoon for a discussion. She knows about my son (my RCH contact had emailed and spoken to her about him) and wants to go through it with me a little more.

I'm so relieved that I'm finally being heard. It's been a tough few years and probably a little of the stress of it didn't help my mental state, but to finally have things happening is just amazing. I'm scared for my son and what may come of this, but also really hopeful that it will help him. It can't hurt to seek advice and I know that there is something "not quite NORMAL" with his behaviour. If he's on the spectrum, he's on the spectrum. He's super smart, loving and amazing really and I love him to pieces. He is who he is supposed to be and there is nothing 'wrong' with him, he just processes and understands things a little differently to the majority.
I do feel a little guilty for letting this go on for so long. I'm desperate to know that I haven't done anything wrong by him and that he is in fact, healthy and happy.
I know things can only get better from here. I want the best for my little man, for both of my kids and sitting on my hands is not giving them the best.

Thanks again for your replies. I will let you know how we get on.