7 week old kitten eating whiskas kitten food - Ok? or not?

Sadly the vet is correct - other than what he did there isn't much else. Are you syringe feeding him? Kittens can be born with congenital issues that you cannot see, and can still decline at that age.

Does he have littermates? Are they still healthy? Mom needs to be spayed as soon as he's done nursing, or passes :( If it's something he inherited from her, it will be passed on to other litters as well.

Get some baby food - the junior meat (chicken or lamb) and mix it half and half with the KMR, and syringe that into him. Hopefully the supportive care will be enough.