I've never heard of that food. I did find it on the net but I still was unable to locate the ingredient list.

How long did you keep him on one food before changing it? Sometimes it takes a little while to see a difference. And you should also change their diet gradually, slowly adding a little of the new food to the old food, within about 2-3 weeks they should be on the new food & off of the old food.

Is he having any other symptoms at all or is it just the upset tummy? When you say upset stomach what exactly do you mean? What is he doing?

I wouldn't change vets... at least not yet anyways.
First I'd call them & explain to them that the problem keeps returning. I'd also ask them what allergy they found and why they thought it wasn't important.
Since this problem has happened quite a few times i'd deffinitely bring him back for another exam & fecal sample.