just being there to sit with him and listen. dont try offering advice because there is nothing you can say that will make him feel better. just listen and reflect back feelings. have you ever heard of reflective listening? you basically say back what he has said and add a feeling. like i can say "i hear you are really struggling trying to find a way to support your friend." i have repeated what you said so you know i was listening to you and i added the feeling struggling to show that i can empathize with where you are at. so things like, i can sense you are really angry, confused, sad that your dad is dying and wont be there to see you graduate, get married, meet your children..,,,etc. you just mirror back what he says to you so he feels like somebody understands. talking about the good times helps as well. what was your favorite thing to do with your dad? what is your favorite memorie? tell me some happy stories? this is a tough situation for you to be in, but your friend has a good friend in you