This is the advice I have gotten:
Listen. If they have a CaringBridge journal going, for instance, make posts that show you are cheering for their son. Avoid the temptation to give advice. Instead, validate what they are saying, letting them know you are listening to them that way, if they are inclined to share what they’re going through. Validate their optimism–a child who seems to be dying of a disease sometimes pulls through! When you send cards, show support for what they are doing and tell them how special they are to you, rather than making suggestions to improve how how they are coping. Above all, try to reach out in ways that will delight and buoy up their child. Especially look for ways to lighten the days that are going to be filled with a lot of waiting around or are otherwise tiring and stressful.
Follow their lead. Avail yourself of the way they want to get news out about their son’s condition. If they post things on the internet, don’t call them up and ask for a personal account. If they have Grandma getting out the news, talk to her, rather than asking them questions. If they have a fellow parishioner running the prayer line, stay tuned for those.
Make concrete offers for help. Don’t just say, “If there is anything we can do, let us know.” If they have a intermediary running these things, go through that party, but in any event make concrete offers, such as doing laundry, cleaning the house, grocery runs, chauffeuring the other children, visiting the parents during waits at the doctor’s/hospital, and so on. If you do offer something as an option, do NOT say, “You ought to do this.” Rather, say, “I’m sure you have everything covered, but if you ever want to do X, I know people in that area I could connect you up with.”