M.,
the best children dentist are the PEDOdontist
pedodontist are trained to treat children, the reason why the choose this field is because they love working with kids, their entire staff are trained to make kids feel as confortable as possible, they have many many tricks up their sleeves to get kids to do what needs to be done in a playfull fun way.
However word of mouth is the best way to pick a good one.
one the first visit they'nt do much to your son, He needs to get use to seing the office and relating with something and not something scary.
At this age your child shouldn't much treatment, infact other than EXAM(the dentist looking for cavities or other problems), CLEANING which is brushing the teeth ,
and Xrays are not possible most of the time at that age.
However if the child needs lotsof treatment, they will explain the options, wich in most cases at this age is problably some tipe of sedation, ussualy is oral where the patien is still awake and breathing on thier own, no needles on the arm.
Also if the dentist wont let you go back with your child to the treatment room, dont worry so much, they'll know if its better for the child to be with you or without you, they know what heir doing.
I hope this helps
L. s.