Whats wrong with my dog serious answers only please

I have a almost 7 month old soft coated wheaten terrier who is has 1 problem after another. When I first got him he had a small infection (like a sinus infection) and was put on antibiotic for it so the yellow nasal dishcharge went away (and it did). Then he was constantly drinking and peeing (though it was diabetes incepidus (dont mind spelling), but it was a bladder infection... which went away. Then i saw these symptoms: constant ear scratching, eye scratching, full body scratching, sneezing, and a runny nose (clear dishcharge). Brought him to the vet and she said its problem an allergy(allergies) so hes been on bendryl (40mg every 9 hours) and he only eats vension, rabbit, fish and no wheat, wheat gluten, rice etc... (Brand Nature's Variety). Well today he slept almost all day, after about a 2 hour nap, he got up and vomitted. then an hour later vomitted again. I called the vet (and yes we have been to 3 vets thus far to get other opinions). Could all these symptoms be linked?

There are alot of things that could be causing these problems in him. Allergies are always at the top of a differential list, but they are very hard to test for. There are skin tests that they can run to see if the dog produces a reaction to them but they aren't always 100% accurate, even in humans.

My first question would be, how long as he been on this new diet? It takes 6-8 weeks of the dog being that diet and only that (no treats, table scraps, etc) to actually see a true difference to know if that food will help or not. If he hasn't been on it that long then just wait it out, I know thats hard but try. If he's been on it that long there are other diets to try (things like kangaroo even if he is really allergic to things). I personally have a german shepherd who has such a sensitive GI tract that she is on a prescription diet food that is very gentle on her stomach and that is all she ever gets. Some dogs even need this because their GI tracts can't handle anything else. It's something that is going to take time.