Give him something you would like to see him wear as a gift.
This would be killing two birds with one stone because you definitely score points for being thoughtful and also you get him to look fashion forward.
Do not tell him you do not like his clothes
Try to avoid negativity; this will only make him more withdrawn to trying out new things. You could instead try to point out that he looks so much better in particular outfits that he has. By all means do not try comparing him to men you consider more stylish than him.
Get the right fit for him
It’s bad enough that his dressing is not great so the least you can do is to have those clothes fit him correctly so that you do not make the situation worse.
Go shopping with him
You could suggest a shopping date where you can encourage him to pick out more stylish outfits without seeming to be imposing.
Compliment him when he wears something you would like to see him in more often
This works wonders. There is nothing wrong with soothing his ego and telling him how good he looks when he rocks that stylish outfit.
Lead by example
Make sure that your dressing is also on point. Give him something to admire and aspire towards so that you are both on the same page. Make dressing up fun so that you can both look forward to it.