So sorry to hear, and I know how hard it must be on you too.
Not sure if this idea would help, but at one time I helped out with a support group with kids, although a bit younger, and not medical related. But they were having to adjust to something they didn't like, didn't cause, and couldn't prevent.
One of the activities was to read stories about kids in different situations, and ask the kids what they might say to the person in the story, to help them deal with that situation. We also asked them what they thought the character was feeling and thinking.
In general, when they described how the characters might feel, it may be a reflection of their own feelings, or how they describe feelings that may give you some insight.
Another thing we told them was that it was OK to be MAD at their situation, and not 'like it', but did need to learn to deal with it. During one meeting we let them yell and shout out if they felt like it - although that did turn out to be a bit crazy and loud.
It's also possible some of his sadness is a reflection of him feeling responsible for some of your sadness and hardship, as I felt towards my parents. My strongest memory was simply wanting 'it' to go away, and to be 'normal', and it just took time.