Depending on what type of chronic pain he has, he really needs to be seen by an appropriate specialist that can help him manage his pain. You also should remember that chronic pain sufferers are highly likely to also suffer from chronic depression. High levels of sustained pain is emotionally draining and exhausting and legitimate cause for depression, so that should be addressed too.
I have Fibromyalgia and several related disorders/issues that have kept me from participating in family events and as it happens, I'm being treated for clinical depression, anxiety, and OCD. Thank goodness for specialists who know what they're doing.
EDIT: For those who have never dealt with having chronic pain, it's nothing to do with making yourself function enough to participate in certain activities. It's just... it's not. When it's a disease like Fibromyalgia or chronic back pain, yes, sometimes you can plod through it but there are days where there's literally nothing you can do. You can't move. Your insides from the bones on out ache and hurt as if you fell off a cliff. Even your digestive system revolts against you. You're prone to headaches. If your back isn't in pain, your shoulders and your legs and neck are. Your stomach might be twisting in terrible knots.
But you know, we deal with it for so long day after day and year after year that we don't complain about it. We build up such a high pain tolerance that typical people would scream in agony if they spent just one day in our shoes. It's why my pain tolerance during labor was so high. But that pain, constant, near constant, isn't something you just "get over" because you love your kids more than you hate the pain. Some days, the pain really is too much.
It's also important to remember that although you have chronic pain and can get through it, your pain isn't his pain. You don't know what he's enduring.