Absolutely tell him. There's two options, let's see what's the worst that can happen in every circumstance:

* He is indeed sick. In this case the worst that can happen is that they will die.

* He isn't sick, you are just imagining things. In this case the worst that can happen is that you made a bit of a fool of yourself. Potentially embarrasing yourself in front of a fellow employee. I guess there's a small possibility of him taking it the wrong way, I suppose.

It's death on one hand versus a personal embarrassment/awkardness on the other. To me the choice is clear.