My older boy is really itchy?

My 29 year-old gelding is really itchy all the time. we used to think it was seasonal, but it's now lasted for over a year. We tried different shavings and they seemed to help but it was so difficult to constantly replace his bedding when the barn does it for us. So we went back to the stuff they supply and he was fine but then suddenly the itchiness came back. He's itchy all over his body: chest, neck, butt, stomach, flanks. And it gets so bad that he rubs himself raw and even tears his skin. We've also tried anti-fungal shampoo but it didn't really seem to help. any suggestions?