I would ask him if it was that he had forgotten
as it might also have been deliberate ( ie simply using lens to keep the hood from getting lost)
It is a simple enough thing which is by no means you trying to be a "know it all, just helpful
I find that if your tone is sincere and friendly most folk without any ego "issues" will take it as meant .. helpful
Some folk are simply brewing hostility and.. best left alone
As an aside ( and not quite related) I was at work with a female colleague whose underpants were showing fairly generously and went up to tell her discreetly. I was told ( in effect )to buzz off..

This was very ungraceful of her( as it was a professional environment, not a social outing) but I quickly realised it was her personality trait at play , not any inappropriateness on my part that elicited that particular reaction
My feeling is that it depends on the person as I would still do i again and not let that put me off being kind
Asking if he forgot to put it on or felt it would not be useful, in a nice way should not cause offence and might either help him. or make you aware he had left it that way he did , on purpose