Please read the Dental Schools Council Admissions booklet, which sets out selection criteria for each dental school.
The only school ruled out is Birmingham which requires a 7 in English. I would also be wary of the QUB which weighs and scores GCSEs and distinguishes 9s from 8s. In addition the Scottish ones and Cardiff are harder if he is RUK.
Far more important is for your son to be clear on what is drawing him to dentistry, getting a great UCAT, A level predictions and achieve at least AAA (ideally A*AA) . WEX is ideally is dental, but customer facing is just as useful.
Just make sure he really, really understands the Uni course and career ahead- it's a 5 year very manual skills orientated training, with a full timetable, heavy workload and continuous exams.
Psychology is a great 3rd a level btw. Good luck to him.