What can I do about my 17yr old, Ali, pulling out fur from his rump,mainly. Tufts fly all over.?

I took him to the dr re: this condition and it was an unknown. Ali was given a cortisone injection into the area. Several years ago he was attacked on the rump but higher near the spine and had to get a series of injections to save him and then he had another altercation. He pulls in the left quadrant of the hip. He has always had the option of being inside or out. Is up to date with all necessary shots. There are no signs of illness other than being more stiff and having more of a challenge jumping. He is spoiled and my husband grooms him daily..even on his abdomen..too much..looks like a cat rug. He gets both a portion of canned (Fancy Feast) and a cup of dry Purina One, daily. He weighs 9 lbs. I hope you can help his bad habit. I thought about benadrel (sp) but that may not be a good idea. At 17, I suppose I will simply deal with balls of fur rolling around the house. Thank you, Connie@ConnieGuy.com