These questions are really a lot less about what specific pieces of information you should or shouldn't share with him than they are about accepting that no matter what you say or do, he is going to do whatever he is going to do--drink, recover, not recover, go to work, spend money, ask about his grandparents, NOT ask about his grandparents, it doesn't matter.
If you think there is something going on that he, as a member of the family, should know, then tell him. Things like the dog passing and his grandfather's illness apply. These are family matters. What he does with this information is entirely on him, and SHOULD BE entirely on him. If he drinks over the dog, then he was looking for a reason to drink anyway, and would have found one sooner or later (probably sooner). If he recognizes a sad situation as an opportunity to practice new coping tools in recovery, then that's fantastic--but those opportunities will arise with or without you as well. You're not making anything happen. It's that kind of thinking that feeds the illusion that we can control others. We can't.
For what it's worth, at his age, I never thought to ask about anyone, either. It's a selfish time even without addiction. You don't have to choose to take that selfishness personally. It's all part of becoming an adult.