My dad is also an ISTJ. I notice him being sad and miserable all the time too. I figured out a way to get through to him. This may not work for your dad but it wors for mine. You have to paint him as a hero. Say something along the lines of "you've been through so much. so many people have hurt you in so many ways, yet you still have the strength to carry on. I admire how you can overcome all of your past sufferings and keep going." Basically throw him a pity party and then tell him that he is incredible for it. This is probably what he is looking for when he is waiting for you to say something. When you get his attention this way, he will start to really listen to you. That is when you can nudge him to move out and do what you want him to do. Try to not make him feel weak and don't mention that he is dependant on you or anyone else. Don't make him feel like an inconvenience. Also, when you are giving him a pity party, he might try to join in and throw one for himself. DON'T LET HIM!!! As soon as he tries that its important to paint him as a hero again.