Clipping is a good idea and changing to a good quality food, as others have said. A food allergy is probably most important. Make sure he has no fleas.
Many years ago, in the '70's I believe, I read a little blurb in Dog World about hot spots. The writer suggests bathing in plain old Murphy's Oil Soap. Later when I had my own grooming parlour, I tried it. Works like a dream. Get some MOS at your local grocery store. Mix some with warm water and shampoo the dog. Leave it on for five minutes and rinse. It will leave the dog slightly tacky. Do this every other day. Then every two days until you see hair beginning to grow. In my shop I've seen dogs with almost no hair left and the owner ready to put them to sleep. I used this trick and in time, the dogs became well again and grew lovely coats again.
I inherited a Shetland Sheepdog with a huge spot on her rear. I used it on her too and it worked. I don't know why it works, but it does.
Lizzie