I saw an underweight coworker

Context
One of my coworkers at the library was overweight when he first got his job. He then went on the paleolithic diet. He got down to a healthy weight and kept on going, so now he is too thin for his large frame. My relationship with this young man is cordial but not close.

Commentary
Actually, "cordial but not close" is pretty much what I go for in all my relationships. I mean, I'm married and I have family and a couple of close friends, but in general I don't bond tightly with other human beings.

And therein lies a problem. I'm concerned about my coworker. He may be ill, but given his personality (he strikes me as someone who sees things very much in black and white) I'm concerned he may be anorexic. But I'm nowhere near close enough to him to ask him about it.

Closeness is not the only issue. Current etiquette dictates that we don't make casual comments on appearance in the workplace, as doing so risks a charge of unprofessionalism or outright harassment. And in society at large we generally don't talk about a person's weight at all. Remember, the only time you should ask a woman if she's pregnant is when you see a baby emerging from her womb. I guess the reason behind this is that we are expected to be hypersensitive about our weight, and if my coworker really does have an eating disorder the last thing I want to do is promote hypersensitivity.

But is the only alternative to say nothing at all? To, in fact, have my eyes open and not open my mouth? Or do I have to take the long way around and get closer to him and then express concern? This loving thy neighbor stuff is hard!

What did you see today?