When I'm in severe pain flares I become like this, just withdrawn and depressed and don't want to do anything so I know how hard it is for him, and you as well having to watch him feel so bad all the time. Like some of these comments have mentioned, doing his stretches with him could help and doing stuff together that he enjoys within his limitations or things the two of you together enjoy could help bring his mood back up. I think that anyone that has chronic illness can benefit greatly by having a therapist that can help through the ups and downs of chronic illness if thats an available option, I'd highly recommend it. Theres always going to be ups and downs with your health when you have chronic pain so trying to figure out coping mechanisms will be a very important step, albeit a hard one, but an important one.