For god sakes tell the poor guy, in some simple yet inoffensive as possible way. I say this as the "victim" of two particularly embarrassing events many years ago that met with "southern" hospitality rather than frank and prompt correction.

My first week out of training in my first real job as a physician I was called away from my work desk/dictation station to do several consecutive procedures that took several hours of my time. I returned to my workstation to find a sticky note stuck to my dictaphone with the following anonymously written on it by one of my new but caring coworkers:
" you have a large booger in your left nostril" and timed only a few minutes after I had gotten up from my seat several hours earlier, and several face to booger patient encounters in between the writing of the note and my finding it. Quick trip to the bathroom showed a sizable now dried and crusty greenie still strongly and quite visibly ensnared in the hairs of my left nostril, exactly as described. I had a meeting with all the coworkers that afternoon and explained to them that no matter how embarrassing it might be to me to have been told face to face there was said booger in place, it was made manifold worse embarrassment by knowing that I had instead walked around with it for several more hours before finding out about it, and that in the future I would appreciate a private but prompt and direct notification. God love Southern Hospitality because barely 1 month later a virtually identical situation arose, this time with my fly open for several hours, again discovered only by an anonymous note on a greatly delayed basis. From then on it bacame a bit of a joking ritual to have one of the techs assigned to "monitor" my grooming actively upon my arrival at work each morning and to perform a full muster inspection prior to engaging my first patients of the day. Needless to say this endeared me to my coworkers forever and immortalized the use of sticky notes for all sorts of pokes of fun and embarrasment. Cannot count the number of times I recieved a "smiley face" on a sticky note with the comment "looking good today" thereafter.

Yes please tell the poor chap and minimize his embarrassment as soon as possible. For all we know he may wear it out that way again one or more times in the future until someone has the heart and gumption to tell him, or he fortuitously finds out on his own.