I had a very similar experience as a child and was terrified of dentists well into my late teens and im 22 now and still cry when they bring out the needles, even though i know it isnt painful, thankfully another dentist built up my trust and i can actually cope quite well now, I'd dump the dentist right away and go with someone else or you'll have so much trouble in the long run.
Not being funny but is the dentist he went to foreign? i only ask because when i had an infected root canal treatment the dentist at the time tried to drill into it with anaesthetic, i nearly shot out of the chair and its was because she couldnt give me the proper anaesthetic because she wasnt trained with the ones they used at the practice.
Sorry for going on abit, hopefully your grandson gets better soon and you find a better dentist