What's the big deal here? Maybe it's an age thing or I'm just weird but if I found out a friend of mine did a weird thing I'd wouldn't hesitate to ask about it.I wouldn't want to know that either. My question is will you let him know you know?......what a uncomfortable moment between friends.
I guess it comes down to how close you are with your pals. With most of my pals nothing is off the table in regards to discussion.
Maybe hes meds are off, maybe he needs meds, maybe he's old and bored and is trying to shake things up, maybe he's been watching a lot of videos of that Dylan guy who thinks he's a chick and it seems really fun to him.
Stop being so stuffy and ask him.