My son is 18 and not dissimilar. He reacted to my diagnosis but since then, he's pretty much been in his own world. Except... When it comes to the crunch, he rises to the occasion. But it does have to be a crunch.
Regularly during chemotherapy I was unable to do anything other than lie in bed. With my absence from the coalface he rose to the occasion more than a few times.
Is there someone who could have a chat with your son? A respected elder, a kind friend? Someone who can tell him to pull his finger out?!
He might be paralysed with fear and uncertainty, afraid of doing something 'wrong'. Much like babies not being 'real' for some dads until they're born, maybe your son won't get it until you're not there/you're not doing things for him/your hair falls out/you're in bed all day.
Feeling unloved by those you love the most is very hard, I get it. My mother rings and asks after everyone in the house but me. It's odd! Sending you big hugs. K xox