It is a mixed declension. http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives
That's a very helpful site. Thank you.
You can't really use dress like that. That's usually used in the context of preparing an animal carcass for food (dressing a turkey, field dressing a deer). You could say "He is dressed in a white shirt," but it's nowhere near as common, at least in my region.
Or you could dress a child or a doll or something, just not an item of clothing.
I suppose you could say "he dresses in a white shirt", but that wouldn't normally be said by a native English speaker. "Wears" is definitely the correct word to use here.
It is a native English sentence to say, "He was dressed in a white shirt."
Usually in English, dress or dresses means something like "She is getting all dressed up for the party tonight". It means that you are putting on a bunch of fancy clothes or dressing up a turkey.