How to get a friend / coworker to wear better fitting pants?

If he is happy then let him be. Maybe he is a pimp in the dating game and that is his game. His tight pants. Personally, its a big no for me. But, he likes them apparently.

This is how I did it for a male friend who was doing the nut cracker thing .

I was at a club and the center of a group of people around

I ask him, Why do you wear such tight pants ?

I like the way they feel, he says

I add, Well, these are as tight as I can go, otherwise, there's just not enough room for, ya know

The girl next to me looked down at my crotch and said, wow, you have a huge penis

The guy realized that I was implying he was small down there, since then he has never worn tight pants and wears em quite a bit more baggier .

Well, the way I can't see why you or someone else should even try to make him buy bigger pants. After all, no one likes to be told how to be or how to dress, right? When it comes to women and clothing there are many views on how they should dress, "risky dressing", challenging dressing, sensual dressing. Yeah, you probably know that better than me :)
But that's my opinion :)

Yeah, if he's your buddy then give him a hard time about it. The old "random opinion" from a female could probably help too. Get everyone to agree that he'd look better in a change of pants. However, if that's what he LIKES, then you gotta let him have his cake.

Assuming you are both straight males:

"Hey dude, did you know you have a penis? "
-Ya
"Then stop wearing your sisters pants and buy some pants that fit. "
-Oh, thanks man.
"Just looking out. "

If you are indeed his friend, this shouldn't be too far fetch as how to keep your bro in line.