If you guys are good friends, yes, you should say something. It's probably just a habit and he doesn't realize how much he's doing it. If it happens so much that he looks like a creepy weirdo in public, you'll be doing him a favor.
There's no need to humiliate the guy, though. Don't yell out, "OH MY GOD DO YOU HAVE CRABS OR SOMETHING?" When he does it you can simply say, "Please stop doing that," or you can make a joke out of it, like, "Would you and your junk like to be alone for a few minutes?" Or you can just give him a look like "WTF are you doing?"
It will be awkward for a second, but worth it. Again, you're doing him a favor. If you had some embarrassing habit you didn't realize you were doing in public, wouldn't you want to be told? I would. It's like telling someone they have a big piece of lettuce or a chunk of ham in their teeth. You tell them because you don't them to look foolish. That's what friends do.
Thanks for the question.
Oh, btw, you have a little white zit on your nose that needs to be popped. No, the other side. Down a little. Yep, that's it, you got it.
You're welcome.
Ha, I had a friend like that. I finally turned and asked if everything was still there?