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I didn't see any comments about whether your dog is eating well, or not. If he does eat the puppy chow, it sounds like you are on the right track. He'll gain in his own healthy time. Dogs like some variety in their diets, but dramatic changes can upset their systems and cause troubles.
My little ***** was never a good eater, but she's always maintained a healthy weight, so I didn't worry about it until I started reading about the things the food manufacturers are adding to the pet food. After that, I started making her food using food from the grocery store (ground beef, brown rice, and vegetables blended to liquid, then all cooked together and frozen/refrigerated). I make a small batch every week. Since I started that a month ago, she eats twice a day -- sometimes asks for seconds, and has put on 1.5 pounds. Obviously, her finicky appetite was for the manufactured food, not food in general.
He's adorable and thanks for adopting him. Our Simon is a border collie/lab mix and his weight is usually between 40 and 45 pounds. He isn't a big eater unless you count the cat food that he sneaks. When he came to us he was eating Ol Roy, but we have tried him on all types of good brands, and he prefers Pedigree. Our cat likes Halo, and Simon likes her food, but won't eat Halo for dogs. He also doesn't eat treats like dog biscuits. Best of luck to you. You will eventually find a food that he likes, and when you do, his weight will increase. The acivity level of border collies is such that he should not have a weight problem. They are so active even if only patrolling your property.