I began sleeping naked in the early 70s and began free balling in 90 then 24/7 by 91.
A lot of guys will wear another guys shorts but that number drops when they know their friend goes commando.
The easiest way to bring up the subject is to ask 'Do you always wear underwear?' when he changes. Some of the younger guys who don't shower at the gym change in the locker room; but, they keep their underwear on so he may be willing to wear a loaner pair.