Maybe it was from treatment. He would be in stress enough to lose its hair. Give your rat the food that would have many nutritional value that would help him to grow his hair back. I would leave it to your vet. I'm not sure about the hair loss. I hope your ratty feels better! (: I had own a rat before, she died of cancerous tumor. I was about to take her to vet to have her tumor removed but she died before her appointment for the surgery. She was fine and but till later she started to get seizures and then she never woke up. My vet felt bad so decided to not charge me. When I told her what happened to my rat and the vet said it could be cancerous or something's in tumor that was affecting her.
I had her for two years and I still miss her dearly, her name was Jebby.