Hi Nacho-
I do not think that the regular Wellness Puppy formula would have caused him to lose weight. I also would not stress too much over under a lb lost in a week. Use the Purina Body Condition System to evaluate his weight as he grows and ask the breeder/your vet what the proper growth markers for this breed is. Each Golden Retriever will be different but there is a still a breed standard.
Wellness is not a bad food, but yes it is expensive. Can take a look at Pro Plan Large Breed Puppy, Fromm Gold Large Breed Puppy, or NutriSource Large Breed Puppy to see if any of those fit your price range better. I will share with you that my number once choice out of the ones I listed will be Pro Plan because of the dedication to large and giant breed puppy growth and nutrition research Purina has done.
