Hmm I'm trying to think of things my friends have done for me.
Write him a card...trust me it really does mean a lot and maybe get him a little gift. My favorite gifts have been really simple, like a fuzzy fleece blanket!
Go and see him and just tell him you support him and will be there for him through the thick and think. But don't talk about him being sick too much because that's REALLY draining on him.