As Faceless said, his spine injury prevents him from sitting for long periods of time, but he has more problems than just his spine like his arthritis and neuropathy. He often talks about his spine flaring up in pain when sitting for longer than 15-20 minutes, but I'm going to guess it's more than just pain flares because the guy gets spinal injections and was stuck in a hospital bed for a month earlier this year. He actually has said he's spent months at a time confined to a hospital bed and even has fears that one day he may wake up and never be able to get out of bed again. I think I remember seeing something about how his issues stemmed from a car crash, but I can't confirm that.