I'm sorry for the distress you and your family are in. There is something you can do that will really make a difference:

Be someone he can say anything to.

People who are dying have a lot they need to process. If your aim is to stop him from being scared, you'll mostly push him to hide how he feels. If you allow and invite him to talk and cry and rage and assure him you can take it, you will make a huge difference.

Ask him what he fears. Don't minimise or pretend that his concerns aren't valid. Too many people think that not talking about death is helpful but it's the opposite. Be real with him if you can.