Hi Sharon, I am sorry to hear about your sweet little Border Collie puppy. You are absolutely correct in wanting to see a specialist. Based on his symptoms you would be best to start with an internal medicine specialist. They have the knowledge and diagnostic equipment to be able to investigate your pups symptoms.
There are many internal medicine specialists – ask your veterinarian for the name of an internist that they recommended or do an internet search for ‘veterinary internal medicine specialists’ in your area.
Thinking of you and best of luck with your puppy.