Oh this was me for years!
Autistic and also convinced my teeth were loose.
I'm slightly convinced one tooth is still loose as it has more motion than the other ones even though the dentist has said it's fine.
I was never really offered any support from the GP for it but they did briefly mention that OCD and trichotillomania for which I have also suffered from can play a part in it.
The trouble is focusing on a sensation that feels uncomfortable can be all consuming when you're autistic due to monotropism and heightened senses.
I sometimes get the urge to just pull my teeth out myself because they're so uncomfortable despite there being no issue or cause.
His dentist should however also check for temporomandibular function as sometimes the nerves around that area of the face can lead us to heightened awareness of the area, and autism can affect how you experience pain and other sensations, making those of us with iffy interoception unreliable narrators when it comes to discussing sensations, ie. Pain for pressure, or not being able to pinpoint the exact location or misnaming locations.
I don't think there's anything you can do other than encourage him to speak to his GP about other possible causes for these sensations.