My daughter lived in an constantly untidy room till she left home when she was in her early twenties.
My wife and I tried all sorts of things, but none of them worked.
Sometimes when my daughter was a work I would go in the room and give it a massive tidy up, so it looked lovely when she came home from work. I hoped this would make her realise how nice a tidy room looks and she would want to keep it that way.
But it never worked and it was soon a mess.
She left home a few years ago and now lives with her boyfriend, and I hoped that would cure her untidyness problems, but it didn't. She is still just as messy.
Funnily enough she is fanatical about how she looks, her clothes, her make up, her jewelery, her handbag, but has no idea how to tidy a room.
I dont think your can force a child to live in a tidy room.
In your case I may be tempted to remove any clothes he leaves lying around and hide them in the loft. Gradually he will run out of clothes and wonder where they all are.
It may make you feel better, but I am not sure it will cure his untidyness problem.
This whole problem is one many parents have and I am not sure of the answer.