We've got similar rules to @diddl - no food in the room, laundry in the basket.
I've learned to fight the battles I can win. I've put lego, magazines, books etc on his bed when he was about 13 years old as I lost patience with nagging - worked for a while.
But I eventually learned to make it into a joke. So I'll walk past his room saying 'get your clothes off the floor soldier' then on the way back say ' dust, sonny Jim, dust!' in a sergeant major voice. It's not worth spoiling a good relationship for in my opinion.
He's now 18 and his room isn't too bad - some of the surfaces could do with a more frequent dust down. His desk? What desk - you seriously can't see it for books, pens etc. He changes his bed and makes it every morning.
Every so often he'll give us the shock of our lives as we hear the hoover going upstairs and think we have a cleaning burglar or someone's cloned our son!
It's not my idea of 'clean' but there's no bugs, no smells and I don't have to slam the door shut if we have visitors so I'll go with that.
Once a month or so we'll have a blitz. The two of us put on some music and he'll clean and I'll supervise - I don't clean for him. He's old enough. It's like he just doesn't see the 'mess' unless I draw a finger along his desk and show him dust! But I don't always like my DH's level of cleaning so does that make me picky??
DS knows how to clean, it's just not a priority yet. This summer it's all about meeting his mates - it's their final summer together - I'm not going to stop that, friends are precious. Cleaning can wait. I imagine, once I've blitzed his room after he goes to uni, I'll be waiting for the day when I can say 'get your clothes off the floor' again.
I was exactly the same at his age and now my home isn't neat as a pin but it is clean enough that, if a friend turned up unexpectedly, they wouldn't recoil in disgust.
As for preparation for university? He can cook, clean, wash and iron. If his room is a mess he'll have to live in it - he knows what to do. And hopefully he'll sometimes hear Sergeant Major mum in his head!
I'd say it's time for a chat about some easy ground rules, a hug and try again. And keep trying - I've learned that one specific instruction at a time is all that some teenagers (and DH's come to that) can cope with!