I would get a second opinion...maybe a hollistic vet... what did your other vet say? What are you feeding him? How much? What treats are you feeding ?
The reason I sak is that Izzy was kind of the same way... She can't have "crap" food or treats.. she tummy has issues with fillers like corn, wheat, etc... We had to move her to a grain free diet...
Pretty much anything you can buy at walmart for dog she can't have
you may have in switch to an L.I.D. food....
It's not normal for a puppy to be that sick all the time...