Well, first off, i'd have my vet look at him and ask if there is reason Weim is not gaining weight? How old is the dog? Does vet also think the dog needs more weight? Do the dog's rib show or what?
Exclusive dog food, is not the worst dog food in the world, is rated above average kibble, here's some reviews:
How much does the dog weigh, and how many cups per day do you feed him? HOw long have you had him?
I'd love to raw feed, but, currently, i can't afford it yet.
I feed my dog Chicken Soup dog food, which is similar to your dog food, but, lists four (4) meats and meals on top of their list, and is not crazy expensive, about $1 per pound, and i top off each bowl with half meat/half kibble. Oddly, i do not have to feed as many cups per day as the bag says i should,
(maybe cuz i add in so much meat?)
and Buddy maintains his ideal weight on less cups per day than he is 'supposed' to get.
So maybe, since my dog DOES hold his weight easily with LESS cups per day than a 50 lb dog is recommended to get, even though he is pretty active dog, with busy life, well, *maybe* that brand could help your dog put on some weight? Chicken Soup is not the absolute top #1 dog food, but, those 'grain free' kind are always chockful of potatoes, postatoes, and potatoes, and my dog gets gas on potatoes. Plus, rice and potatoes are almost the same so far as nutrition goes.
I also add in small dab of cannola oil or fish oil capsule, a few times a week, for his coat.