
Hi Bunty,
In an adult dog I'd recommend a 500mg tablet a day of Evening Primrose Oil, not sure for puppies as it's high in Vitamin E and I'd be a bit concerned about oversupplementing. Instead, I suggest your sister add about 1/2 teaspoon of a good olive oil to his food. I have two Cairns and they both tend to get a bit itchy when the heat goes on in the winter. The Evening Primrose Oil fixes the problem.
There are lots of things besides dry skin that could be causing the itching - including reaction to perfumes in laundry detergent used to wash his bedding or any other chemicals used around the home.
Also - if your sister baths her puppy, please tell her to stop. Cairns do not need to be bathed - ever, unless the roll in something stinky and that's literally the only way to remove it. Bathing can easily cause dry skin and itching - not to mention itching caused by the shampoo itself. My three year old Cairn has never been fully bathed - although I have had to "spot clean" a few times because of rolls in fox poo and various other disgusting substances. My almost 3-year old I bathed a few weeks after I got him at 1-1/2 years old (he had a very strong kennel odour) and never since.
Unless your sister purchased her puppy very recently from a knowledgeable breeder a 4-month old Cairn should be loosing its puppy coat. (Most breeders remove it when it is ready.) The puppy coat is sort of a thin, fluffy looking long outer coat. This all needs to be stripped out by hand - it the puppy still has most of this coat, it could also be contributing to the itching.
Lots of people do not like Eukanba, if your sister tries all of the above and after a month the puppy is still itching than I'd suggest switching foods. If she switches, choose something that is labeled as hypoallergenic (James Well Beloved, for example) and select a mix that does not contain the same protein as Eukanuba. So for example, if the Eukanaba's main source of protein is lamb than pick a food where the main source of protein is chicken.
Stacey