DD(11) has Autism (aspergers dx, when it was still called that). A few thoughts that may help. (I am only on page 1 of the thread yet so apologies if you've moved on or someone else has already said these).
Teeth - we make it part of the routine before bed, it is necessary, we constantly ask about having done "wees and teeth". Extending music from toilet to teeth makes sense. Also - does he hate the toothpaste? DD hated mint, really hated it, for a long long time so we were using the bubblegum or strawberry flavoured toothpastes for young children up to about 6 months ago - we've only just moved on to mild mint versions.
I know you can get some toothbrushes that are soft plastic that fits on a fingertip - would that work better than a proper brush?
Would he tolerate a swish of mouthwash to help as well?
Also, do you use timers at all? Would he accept needing to brush for 60 or 90 seconds until the timer goes? (School dentist was very strong on it to DD, about needing to brush for 2 minutes, so she was better after the professional told her - and we use timers a lot anyway for all sorts of things both cooking-wise and DD-wise).
We're lucky as, despite the dodging that we've gone through, DD is reasonably good at brushing teeth. We also have a really good dentist who is great with DCs - DD had to come with me a few times when she was small so saw me getting checked, then another visit lay on top of me with his special sunglasses and saw his light and mirror and he had a quick look, then on top of me he showed her the air puffer and water puffer and had a better look, and eventually she lay on the chair herself with me sitting beside her feet and my hand on her legs that I was still there.....could you get back to first principles to reduce the fear?
Definitely pursue the back pain - we've had times when there was nothing physically apparent but DD was in pain, we've had it all checked out and she has understood that, but sometimes we actually found that when it was being followed up, and she was getting pain relief as needed (on medical advice), other anxiety or stress things were coming out and as we had time to talk about them, her stress levels reduced and her pain would reduce as well - unexplained belly pain seemed to be anxiety related but she also has actual problems with recurrent ear infections showing us she has a very high pain threshold (not saying anything a few times until the eardrum had burst).
When she was having a LOT of meltdowns, particularly in school, one thing that really helped her cope was to reduce her overall stress levels rather than trying to address the specifics of the meltdown. Sounds odd. But it's a bit like, she had a petrol tank of ability to cope - when she was stressed in general, that took a lot of the petrol in the tank so she only had a small bit left over to deal with any new thing happening so meltdowns came very quickly. But when her overall life was calmer and more relaxed, she used less petrol dealing with the everyday stuff so had more petrol when she needed the boost from someone else winning or hearing an extra loud noise that hurt her ears or having to do something difficult in class or a bully getting in her face (yes we've had issues in school with that too!)....
We did things like tried to make sure that when she was having fussy times with food, we didn't focus on that but made sure there was some food she would eat (and she still has a large drink of milk at bedtime - sometimes hot chocolate now, almost always warm, but that's as much about getting calories into her as an aid to get her to sleep). Reduced choices down to just 2, and sometimes even just presenting a fait accompli when making a decision was beyond her (although working on decision making when less stressed). She was allowed to go into a quiet space away from the bustle of home life, cool and dark and calm, either to chill normally or especially when meltdowns were happening. We worked on relaxation techniques with her, and did things like rubbing her back (soothing) or letting her have long baths and playing with her rubber ducks (water soothes a lot - but sometimes showers can feel like needles on her skin so baths tended to be better). We tried to have clothes that were comfortable to her - soft tracksuits, cut out labels, seamfree underwear etc. Fidget toys to get anxious energy out. And reassuring her that we loved her lots - spending time with her, getting her to help with chores working together, reading to her in bed even long after others her age were not getting stories, even now she listens to audio books or talk radio in bed, that sort of thing.
We've also tried to talk to her about building her own coping mechanisms- so knowing to go and tell an adult in yard if there's a problem rather than acting out when provoked, telling us that she is having a problem, learning to walk away, do deep breathing, take some rescue remedy, do a scrawly colouring picture to get anger out, run a length of the pitch to get too much energy out....and sometimes about breaking things down into smaller chunks. So do 1 problem of maths at a time, typing her spellings or poems into her laptop as a way to help her learn them (she seems to be a "rote learner"), having a few minutes to play a game (on a timer) in between some subjects before coming back to do something else in homework, allowing her to do reading curled up in a comfy chair not straight at the table, or sometimes getting the book on audiobooks to read while listening to it (particularly when the reason to read it is more about knowing the story to discuss it as a group in school than actually being able to physically read it) or finding it on my kindle so we can increase the print size (while she's well able to read, she seems to have problems with large amounts of small print text).
I hope that there's something there that's useful.