Any veterinarians on here?

I am fostering a dog from the local animal control, and over the past week, the skin on his back at the base of his tail has began to get nasty and gooey, and his hair is coming out in little chunks. I have bathed him twice since I've noticed it, with dawn dosh soap, and haven't seen any fleas come off of him. I'm worried about him, because he's so itchy. :(

Can someone please help me?