Newly diagnosed 5yo. How can I support with medical appointments?

My 5yo has recently been diagnosed with mild autism. He is more than happy to attend appointments but as soon as we get there he won't allow them to go near. 

He's been needing a tooth removed since April and has been deferred twice due to non compliance.

We have a dental appointment Tuesday is there anything aside from words of reassurance I can do to help him? 

We also have a upcoming eye appointment which they will need to apply eye drops. He had this before which really upset him so need to make him aware of what's to come but without filling him with fear!

Doctors also are a no go - my GP told me previously "well if he don't allow me to examine him there's nothing I can do" then proceeded to prescribe antibiotics which weren't relevant....!

Thanks