By your own admission, your manager is already aware and shares your concern. I would simply leave it at that. The reality is that regardless of how valuable he is, there's probably nothing that they can do. If he chooses to leave, they can't really stop him.
So let's look at the positives and negatives:
Positives
- Management is even more aware than right now that he's leaving.
- You demonstrate your concern for the welfare of the company
Negatives
- You potentially annoy your manager by looking like a pest since they already know he may be looking.
- You hurt your relationship with your coworker which could have an impact later on if you ever want him as a peer reference.
Given that they already know he is likely looking, I don't see what there is to gain, to be completely honest.
