Well, you could try getting him some new clothes... try to stay positive about it; criticizing doesn't ALWAYS work... depends on the guy.
One of my "girl friends" went on the attack about MY clothes recently and many of the girls at the table seemed to jump in, complaining about my look.
Now, a bit about me... I wear nice dress shirts, so it's not like I'm wearing a holey A shirt and stained jeans. I'm a "business professional" so I take care of my hygiene fairly well naturally; I'm no metrosexual, but I make sure the teeth are brushed, I have good breath, I smell nice, and I don't belch, or scratch myself in public.
While I'm not super fashionable, I do alright. I know the basics. I have an artist's background, so I would think that would account for something.
Their attitude was complaining about everything. But I've had plenty of times where I've met these girls with a look I thought they'd like - I "clean up" a bit differently, they barely seem to notice...
My first tip... ladies... if you don't like a guy's look, you can be honest, without being nasty. But if you DO complain, and he finally changes it, then COMPLIMENT the guy! If you still don't think iit fits him, find something good about the guys look (again, you don't have to lie), and focus on duplicating that with a different look again (Keep in mind if he's spending hours on this and hates it, you're not doing him any favors by continuing to shop).
2) Complaining may not make me change my look, especially if others seem to love it... but if I do change the look, and no one seems to notice or care, then I'm going back to the look (I like) that gets me more compliments.
3) Get other girls in on it (NOT his mom). DON'T get a group of girls all at once to drag him to the store, unless he's OK with it. A group of girls is usually dangerous (With my friends, a group of girls talking is like letting my brain cells die a slow, painful death). What I really mean is that if they won't speak up just naturally, get them to speak up. Obviously, they don't have to flirt, but a quick compliment from a lot of girls will go far for keeping a new look.
Also, if there's something he wants to change about your look, let him do it - fair is fair.
4) Don't make it too hard for him. Don't drag him clothes shopping for three hours where he can't get a break, can't sit and just "let you do your thing."
I hated shopping with my ex because she'd drag me into women's stores and demand that I agree something was cute or icky. Then she'd talk about everything over and over.
Consider making it into a hunt type thing. Look for one type of thing, get in, get out, and reward him. Let him go to our equivalent of a "toy store" (Let us play with the power tools); get something to eat... at his favorite bar...
Find the things that make a guy fine with shopping and try to incorporate that.