Yes I have some ideas!!
I had this problem with my son(now 18) and I took everything except his bed out of his room. When he decided to keep it clean, I let him have things back. I did not allow him to play his video games in my living room, so the only place he had was his bedroom and without a tv you can't play video games or watch tv. It took about a week and that was it, now I can't say he kept it totally spotless, but at least the smell and the clutter was gone and now he keeps his room orderly and livable!