This may sound horrible, but my first thought is that he's allowing himself to feel sorry for himself.
I can't (and honestly don't want to) understand what he's going through w/ the knee & the cancer, but his actions sound like a little boy throwing a fit for attention and/or feeling sorry for themselves. My S7 did this the other night -- said he was crying in church b/c the pastor said something about giving your mom & dad a hug and he can't hug his dad. He said "grandma saw me crying and didn't say anything." In other words, he was wanting a reaction, not that he was just honestly sad.
There again, to me by not talking about it, it makes it an even bigger deal instead of talking about it, talking about how he's feeling and dealing w/ it.
Or maybe I'm just way off base and he's just trying to figure out what/how he's feeling and how to deal w/ it all and just isn't ready to talk about it yet b/c he hasn't gotten it figured out.
Those are just my thoughts when I read your post.
Me: 38 H: 35 S4, S5, S10 Bomb 01/07 Wanted D - nothing would change his mind Numerous A's prior to D bomb; EA prior/during D bomb Piecing 04/07 Deployed for a year 05/07 Still Piecing 2010 M 11 yrs 05/10