Welcome to the bb forum and thank you for sharing your story.
You sound like an incredibly caring person and a good friend to your fellow DND game player. Sounds like your friend is having a really rough time and that the game is likely to be important to him, given he attends when he can and has a good time when he presents.
I do, however, understand the impact of his frequent absences on you and the others. Can you imagine how upsetting it must be for him? To not be able to participate in something he enjoys due to illness. So sad.
I'm wondering if it's possible to talk to him and see if he wants to take a leave of absence while he recovers from his illness. It's possible that the "pressure" of having to front up and then the disappointment of not being able to do it is troubling him, too. If he were to take a break that would alleviate the frustration for you and the others cause you already know he won't be there.
Would it be possible to fill his spot on a temporary basis? Can you play one person down for a spell? You could set a return date in a few weeks (take your cue from him on the timeline) and then if he still can't play talk with him again before moving to replace him.
Kind thoughts to you