My sex partner has a tumor.

So I've been having causal sex with this guy for about a year and half now. We're pretty good friends, I like him and the sex is amazing. Well he recently found out that he has a tumor near his brain, and is having it removed. We met one last time before his scheduled surgery, and it was probably the best we ever had. But now he seems a little different, or withdrawn. Of course it's not love, but I do care about him a lot, and I want to be there for him through this. But I'm not getting the same feeling from him. I'm really confused how to handle this situation.