The attitude you have described is not good enough, however if you were not happy with the car or the way the sale was handled then you could have made that clear at the time, which if not resolved then would end-up with cancelling the deal.
The service is not good enough, but your son chose not to inspect the car in advance or upon arrival, and subsequently accepted the car after the two issues were identified and not fully resolved. There was plenty of opportunities to stop proceedings.
Sorry if you feel that’s brutal but that’s an objective review of the scenario you have described. Very frustrating I’m sure, but by accepting it without agreed resolution you have ultimately accepted it which reduces your leverage with the dealer.
Like leaving a restaurant dissatisfied with the food or service, without mentioning it, and then emailing head office. By accepting the food/service it just have been acceptable, and not mentioning it prevents the restaurant remedying the issue.
Like I say, apologies if that’s not what you want to hear. Wishing you and your son the best in resolving the issue, but most importantly don’t let it sour the purchase of a dream car - those opportunities are too few in life, so don’t let it be ruined