People react to scary news in different ways. Some will listen and be polite about receiving the news from others, some will react like your h did and others will go into panic mode. If this type of situation occurs again, just say "I'm sorry you feel that way" and hang up. You do not need to justify why you called. You did the right thing in notifying him. When he calms down, he may very well apologize. Some just sweep the episode under the carpet.
Right now, he doesn't really know how to deal with the news about his grandparents. You have to remember, he's in crisis still and they react to news, positive or negative, differently than we do.
Continue as you have been. Just be yourself. You did nothing wrong in contacting him. He needed to know.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
