Generally, when it comes to responsibility for sharing privileged information in the workplace, it's best to respect the management structures in your organization.
In other words, unless you're this person's boss, or you have some official responsibility for his performance or his employment, or you've been told by your boss that it's OK to share the information with him, it would probably be a mistake for you to tell him anything. It seems that none of these things are true, which leaves you in the position of putting yourself at risk if you tell him. None of us can decide if you want to take that risk.
You may not inherently be in a position to even consider this as an ethical issue: you may not be privy to the entire picture, there may be other factors at play in this decision which could change your perspective on the "ethics" of telling him.
That said, it is a little awkward for your boss to share this with the team. If this seems to be a pattern with your boss, and it makes you uncomfortable, you may want to consider that if or when you consider looking for another job. But, you should separate the question of, should I tell my coworker about this? from the question of, is my boss acting in a way that makes me uncomfortable?