There is a friendly homeless cat that we have been feeding and looking after. He must live outside mostly, I don't think he has a home. He has some hair loss on his back and up the bottom of his tail. His skin underneath the hair loss is somewhat pink. Any ideas what it could be? Any suggestions for helping him?

It could be sarcoptic mange.
It is caused by a hair mite which also infests dogs and can also infest humans.
You should take the cat to a vet for a diagnosis and treatment. In the meantime try to keep the cat away from your other pets if you have any.
http://en.wikipedia.org/wiki/Mange

Could be several things ranging from fungal such as rig worm to the most probable cause, cat fights. Get him looked at by a vet if you are worried.