Mine did this recently - but just after meals (and like yours, undigested). I took her off the painkillers and raw tripe and that solved it. Awful that he's traumatised by the vet, but if it's happening regularly I think he needs to go. If mine didn't get better by itself then the vet was going to do blood tests. Good luck.
We have an 8 year old GSD x collie, who we rescued off a friend 3 years ago. Ddog was not fed properly before we got him and has always had a very sensitive stomach, but has been settled on James Wellbeloved Senior mixed with tinned meat for 2 years. We mix dry with meat and split it into 2 meals, morning and evening. (he wont eat dry without the meat so vet says it is a good compromise) He normally has a supplement in the evening to help with his joints as he has mild arthritis. He also suffers from horrendous car sickness and always has.
Recently, he has been vomiting up his undigested food, about 2-3 hours after his meals. This happens a few times a week. He has no diarrhoea and seems otherwise fine in himself but gets very upset when he is sick. I'm loath to take him to the vet as it is very traumatic for him, and I don't really want to start changing foods as it took nearly a year to find something he would eat without problems. Anyone got any ideas what could be causing this?