I’d just say u gotta let it all Out and be honest to him as well. Show him u care and yousee him struggling and how sorry you are and how u want to help him . Tell him you’ll genuinely be there for him and show it. If that means forcing converstaions or leading them completely of that what it takes then u gotta do it.
Just say, in person and in private, something like“hey I know with everything going on with your dad, you must be having a really hard time at home and dealing with the day by day. I just want you to know that I’m here if you ever need to get out of the house, or call, or vent, or whatever. Just lmk how I can best support you. AND LEAVE IT AT THAT! He most likely will just say “k, thanks”. Just do know he got the message.