I am sorry to hear you are worried about your Yorkie, and so am I. My name is Rebecca. I am a veterinarian and will do my best to help.
You can get a product called Capstar; that will kill all the fleas on him, is very safe, but only lasts 24 hours. You need to put a good, safe, long lasting flea product that keeps killing fleas on him, such as Advantage, or Frontline, or the Seresto collar.
The yellow color to his tongue is extremely concerning. Do the whites of his eyes look yellow? This could be jaundice and that can be very serious. It could be that that fleas have sucked so much blood from him that he is anemic, or that the fleas have infected him with a disease causing the jaundice. Or he could have liver disease, or a serious blood problem called hemolysis.
He really needs to see a veterinarian as soon as possible if he is jaundiced. This could even be life threatening.
Let me know if you have more questions I can answer.