Hello and thank you for your question.
How frustrating this must be for you! What does he say when you ask him why his place was always clean when he lived alone and now he is so messy??
Okay thank you for the additional information. It allows me to know if this is how he always has been or if he changed, expecting you to be the one to clean. It is the way he is, however he is certainly old enough to understand that he is messy and needs to change and help out with the housework.
You have already tried so many tactics and it's bewildering that nothing has helped.
You could ask him what chores he would like to take care of and allow him to pick. And then make up a list with a schedule and that when the chore is done, he puts a check next to it. If he can't pick then you divide it up yourself and again with the checks.
If he does clean up, be sure to tell him that you appreciate it. However, if he doesn't change at all and just doesn't even try or only shows a minimal effort, I would let him know (and you may even want to let him know now) that if he doesn't put in the effort, this is a deal-breaker for you. T
hat it actually shows not just that he is messy but more importantly that he doesn't seem to be able to reach any kind of middle ground or take your feelings seriously and show through behavior that your needs and happiness matters too! It could show up in other areas I mean where he is going to be stubborn over changing or reaching a middle ground.
So in summary don't hesitate to let him know exactly how much it bothers you and that you are seriously considering that this may be a deal breaker. Then see if that makes a difference to him.
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Okay so this does go deeper than just him being messy and is more about him (maybe) exerting his own will and control. It could be that since he couldn't rebel against his Mom, he can now do that with you, but this is misplaced and won't work.
You can let him know that your relationship is your own, not his parent's. If you are already feeling like you need to tread lightly or he will shut down, then it sounds more like his issue that he needs to resolve on his own, and it's about more than the cleaning etc. Like when he said, I guess I can't do anything right. Be sure to tell him the things that he does that you really like etc. and you can even ask him (gently) if maybe he is worried that you will boss him around like his mom did to his dad. And if that has anything to do with his resistance. His avoidance and leaving the room are okay but only if he returns to then talk about the issue and not just hope that in avoidance you will just let it go.
He has to see the consequences and then maybe that will make him change. best wishes!