My cat is 15 years old he has oulled out little clumps of his fur for years but recently he started pulling out so much that he has a large bald spot on his side.

We got two cat that were 6 years younger than him about 3 years ago. they never really got along and he was always hiding from them. They moved out for about 6 months then moved back in in the beginning of May. One of the younger kitties died last week. he didn't strt to pull out the big clumps until about 2 weeks ago. could he be reacting to the changes? Or could it be something more serious?