If I remember correctly, the FAA, by statute, must investigate. If he's in denial and has something that causes night blindness, it could be a simple vitamin A deficiency and easily corrected. It could be something serious that he needs to know about for his own sake and for the sake of all the people he may kill. It is well to remember that both he and you have greater responsibilities in this than "only" yourselves.

Or he could be perfectly healthy and you are completely off-base in your analysis.

First of all, are you usually wrong or right in your judgement concerning other pilots? If you tend to over-react (like normal people do) to pilots when off the ground, then you might let it go unless you feel it is very compelling.

If you are usually right then you have a problem because the odds are that you are right this time.

How about a decision matrix?
Here are your choices:
If there is NOTHING wrong and you FAIL to act then nothing happens.
If there is NOTHING wrong and you DO act then what are the consequences?
If there is SOMETHING wrong and you FAIL to act then what is the worst possible outcome and how would you like to live with that on your conscience?
If there is SOMETHING wrong and you DO act, what are the consequences?

Do not investigate this yourself. You will start rumors and that may expose you to legal action, in which your gossip undermines his ability to work. By going to the FAA you are protected to some degree, I'm pretty sure.

If you're still unsure, you must have contacts in the FAA. Talk off the record in a "just suppose" scenario and see what happens.

If I were you and it was my pilot (a colleague whom I like personally, and trust completely), I would tell him outright. Also I'd tell him if he didn't get checked I would drop a dime on him. But I know my guy really well and his ego is normal pilot size (very big, but not pathological). He'd do the right thing if I brought it to his attention.

B