my puppy tested negative for mange, given antibiotics, but still looks like he is losing hair and has sores and is itchy. ?

Have you possibly checked him for food allergies? Depending on his food it may contain wheat/corn...

Fleas
Maybe he scratching so hard he's getting sores and hair lose

Or has he been in a fight u might not know about

We only give him natural food with no corn or wheat because it is bad for great danes. It all started when we would bathe him in baby shampoo. He would break out into a rash and then bumps on his armpits and private areas, so we switched to more sensitive shampoos, but it's almost like he;s allergic to the water. : (