Advice on many food problems

[Dylpops]

Hi I have 5 year old mixed breed. His good weight is 20-21kg but he suffers many food problems which cause weight loss, itchy, chewing paws, chewing tummy, soft poop, hard poop and every colour variation of poop.. And generally an unhappy little lad.
What he can't manage is a repetitive diet, mixed protein, too much wheat, too much grain, too much spud, peas, maize. Not great on poultry as a protein source and fish as an alternative isn't great either, nor is beef, boar, rabbit, reindeer, goat, lamb, pork - I think you get my drift!!
We thought we had finally gotten to the bottom of the issue when we went to single source protein and did a weekly menu that avoided the same food being repeated. He managed to get back to a good weight, stopped loosing hair, no chewing or licking. It took 6 months.
However out of nowhere we have an unhappy lad again, weight dropped, chewing, licking etc. I am now at a loss and the vet hasn't been that helpful saying just cut out the food that makes him ill - he can't live on fresh air as most things do make him ill. I've tried raw and tripe which actually made him lose weight and he also started eating his poop. I've tried alternative protein such as ostrich but can't feed on repeat as he gets ill after he has had it for too long. It makes me so upset that I can't keep him stable and happy. Am I missing something? Is there some illness that does this? Any suggestions PLEASE!!!!