Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
How long has he been showing these signs?
Any changes to the skin (ie pustules, pimples, redness, crusts/scaling, ulcers, or sores)?
What flea treatment did you use? When did you last apply it?
How long has he had the diarrhea and diet change before it started?
Can you take a photo of this to let me see what you are seeing? To post a photo, you can use the paper clip button on the tool bar above the text box. Or if you cannot see that on your phone/computer, then you can post them on any site (ie Flickr, Photobucket, Imgr etc) and paste the web link here for me to have a peek.