If he's a real friend, and not merely an acquaintence or co-worker, you ought to be able to make the point very directly and without embarassment to either party if you approach it as a joke.
In my experience, people take the most well-intentioned criticism quite hard (and often defensively) if it is offered in an earnest, serious tone. OTOH, anybody worthy of being your friend will have the ability to laugh at themselves.
One non-clothing-related anectdote springs to mind. I have a friend with whom I have played golf, off and on, for several years. Although he loved golf an almost unseemly amount, he was simply not very good at it. One of his great faults on the links was his posture: before every shot, he practically squatted by the ball, hunching his shoulders, dipping his knees, and curling his pelvis and lower back towards the ball. Many of us admonished him to "stand up more," to "keep a straight spine," etc. To no avail. He simply wouldn't listen and got defensive. Finally, I said, "God, man, it looks like you're getting ready to f*** a sheep!" After he quit laughing, he began working on his posture in earnest. Not long afterward, he did break 100 for the first time, which was a vast improvement over prior scores.
IOW, tell him, in the most offensive terms you can come up with, that he looks terrible. Just smile when you say it.