What happens when you do talk to him about it? Does he see that he's being unreasonable and promise to change, then just slip back into his old ways? Or does he blank you out or tell you he doesn't see the problem?
In terms of what you can do, I think they've mostly already been mentioned -
Don't clean up after him, unless for DCs safety. Even then, just move his mess to somewhere that causes him a problem instead.
Call him on it. When he mentions clothes on the floor, say "Maybe both the irresponsible people in this house need to spend the next half hour picking their clothes up off the floor"
If you can't stand the mess, put all his stuff in a bin bag and put it somewhere out of your way, and preferable out of sight.
Don't wash anything that hasn't made it to the washing basket.
Basically, make him the one who has the inconvenience. Most people won't bother to change their ways if they don't feel the pain. Make it his problem, he'll be more likely to do something about it.