
Hi labmad,
quite simply if he has loose stools he will lose weight - whether this is through change of diet or possibly over feeding (quantity required may well be very different from one manufacturer to another), if the food is firing through his GI system rather than being slowly digested and nutrients absorbed then it is doing him no good.
I'm not sure of the merits or otherwise of Iams but would be surprised if moving on to Beta was an upgrade ..... that said I must admit I don't understand the fashion for new owners to wish to change away from the diet the breeder has produced a healthy puppy on unless of course said new owner has in depth personal experience of certain foods being less than satisfactory. Even so, why then buy a puppy from a breeder feeding (in the owners opinion) low grade or unacceptable food?
Pups have enough stress to cope with being parted from dam, littermates and all familiar folks and surroundings without having the added pressure on their delicate systems of travel, change in water and a change of diet - however slowly done. IMO your sister would have done better to stick with the original diet the pup's system was used to and, as with any growing puppy, slowly increase the quantities per meal.
Was the puppy used to two meals of scrambled egg a day before your sister took it on? Personally I give very limited egg to dogs of any age (except for those fed raw which enjoy yolk, white & shells) other than as a gentle reintroduction to eating after a period of fasting.
Has your sister contacted the breeder for advice and also has she had the puppy vet checked?
Hopefully there is no serious issue but the breeder should be advised and also the vet.
regards Teri