You are his woman. You are attracted to him, and he is attracted to you. This is perhaps your greatest strength. He wants to look good for you.
So tell him not what clothing he should wear, but what you'd love to see him wearing. Imagine, if you will, walking hand-in-hand through the mall and passing Victoria's Secret -- which would you rather hear: "You really need to get sexier lingerie," or "You would look really sexy in that"?
Instead of "You need to get some better-fitting pants," try, "I would love to see your butt in those jeans."
Manipulative, yes. Malicious, no, and the right pair of jeans really can do wonders.
On the other hand, pointing out what his butt looks like in his current, ill-fitting jeans is probably not the way to go…