Coarse Fur?

My cat is about 3 years old, and he's my baby, I love him so much. I just moved into a new house in March...and I've changed the food from Special Kitty (Due to the scare from them) to Purina One:Chicken and Rice Flavor. He was a stray that I found in the trailer park as a kitten, and his fur has always been somewhat mangey, but once we moved and he had a food change his fur became nice and soft and shiney. But about a month or 2 ago his fur became coarse and seems to be...overly thin (he's been shedding alot too). Is it maybe the cat food? Or is it something else that is making his fur coarse and fall out? I love my baby any answers will be helpful!! Thank you!