To be a little more specific, in your case I think a card with a brief family update, a picture or two, and a msg. briefly telling him what's new with you and your family. Then mention that you wanted to let him know about your dad's health issues and your concerns about his emotional well-being as well.
That it would be such a tonic for Dad to have a call or a visit from him. I wouldn't try to guilt him the first time I reached out. Just let him know how lonely Dad is and a reminder that life can be so hectic that sometimes we forget those closest to us. Presume that it's just an oversight on his part. Take the high road.
If he responds -mission accomplished. Give it a couple of weeks or so. If no answer, then use a couple of Scriptures as a reminder to him.
If still no answer, your brother is what he is and there is nothing you can say to change him.