The the severe itching in the area you describe could be a few things.
Dogs have anal glands, just inside the rectum, that are used to mark the territory when they have a bowel movement. Sometimes the drainage ducts become blocked. Then the glands become too full or infected. It is quite painful and annoying for the dogs. The treatment is to have the vet, or veterinary nurse, drain the anal glands manually with a finger in the rectum. If there are signs of infection, antibiotics will be needed. If there is inflammation, a short course of prednisone may be needed.
Fleas like to bite over the lower back. That is the favorite place to feed. One flea bite can cause a big allergic reaction. The fleas always go on the dog to feed and then they jump off. So you may never see the offending flea. Shampoos wash off the topical flea products as well. You can apply the frontline every three weeks if needed.
He may also be having pain back there. He is an older guy so he may be having some arthritis
in his hips or spine. You can try giving him one of the small dose aspirins which come in 81 mg tablets. Give it with a meal to help prevent gastrointestinal upset. If it helps, then I would take him to the vet and get him a pain medication that is more effective on pain and less likely to cause gastrointestinal upset.
You can also try 1/4 of a 10 mg zyrtec tablet every 24 hours.
Or 1/2 of a 4 mg chlopheniramine tablet every 12 hours.
Sometimes a different antihistamines is more effective on certain dogs.
You an can always spray topical hydrocortisone spray, on any itchy areas, every few hours.
I hope this helps.
If you have additional questions, please reply.