My daughter with CF is 21 so I can relate a little bit to what you're saying. I can't imagine the frustration you're feeling at this point and there are no easy answers. It is very typical to see a huge decline in self care with adolescents for too many reasons to detail here. Suffice it to say, he's in a firestorm of developmental stuff that teens normally face, complicated by mortality staring him in the face, PLUS whatever else is going on in his life socially, spiritually, mental health-wise, etc. It is a VERY heavy load to bear.
I would think of someone in his life that he loves and respects - besides you and your husband. Share with them what is going on and ask if they would be willing to talk to him. That can happen in a number of ways - that person alone with him, that person joined with some or all of his loved ones, that person with him and his doctor or with you all and his doctor. Everyone is different and you'll have to consider what might work best with his personality as far as what will really move him. The biggest thing is to really LISTEN to what he has to say. Too many times, we are so worried about losing them that we jump in and discount all the reasons they don't want to bother with treatments, etc. It was really, really hard for me, but I've learned to be somewhat better at putting myself in her shoes and trying to understand how difficult just daily life is for her with all that she has to do. I find that when I do that, when I listen and sympathize and don't add the "but...." - she is much more open when at another time, I sit down and say, OK - we've got a problem here, I know (insert all that she's told me about her feelings here) and so how can I help you make this doable? What ideas do you have? That seems to work pretty well. I also try to remember that the quality of my relationship is most important because time is so precious. Do I want to spend that limited time fighting with her all the time or do I want to try to understand what she is feeling and thinking? It's the hardest thing I've ever had to do, I don't do it all that well I don't think, but I try to remember it as my goal and it has helped. I pray that your grandson is able to feel and know how much you all love him no matter what.