Bunnybb it was the OT who recommended ours, but apparently you can be referred by any health professional, so GP or HV can refer him for you.
Ds1 has only been to the dentist once, when he was 7. 
Having spoken to the OT about him as well, she said the GP will refer him to the SN dentist if I request it, so we have it on the evergrowing list of things to organise for him. Apparently the SN Dentist here is great. The surgery is deliberately made to look less intimidating and they are happy to do just a visit and look around first time, then a sit in the chair the next time and gradually build it up from there going at the child's pace. They also have different tools available eg a drill type thing that doesn't make that awful whiny, screeching noise and for that reason, plus the whole sensory thing about having someone's fingers in your mouth, it's particularly appropriate for children who have ASD.
I was planning to try disclosing tablets with them to encourage better brushing, but ds1 has recently developed an obsession with brushing several times a day, as he's paranoid his teeth will start to rot if he doesn't. No amount of explanations or reassurance seems to work (he's the same with hand-washing) so I daren't show him how much actual plaque is left after he brushes or he'd spend all day in there brushing. Thing is, despite all the brushing, his teeth never seem to look clean. I think this may be down to enamel damage/mottling from his gluten issues going undxd for so many years. I'd like to get it properly checked by a dentist and discuss whether there's anything that can be done to improve the appearance of at least his front incisors.