I'm glad to hear you got there eventually. Once he has the skills and he knows the process you can incentivise keeping the room clean. Maybe have room inspection at a certain time each day and if it's up to scratch then reward him with something he cares about. Spend time giving him pointers and help him through the process. I see this kind of thing in my line of work. 17 or 18 year old clients just don't understand things that aren't explicitly laid out for them step by step. We all know what clean your room means but they don't know how to get there. Another thing I've noticed is that if someone has a certain amount of space and you give them one item more than they can fit, you've given them an impossible task. We would all kick back at work if we were given a task like that but sometimes we give these tasks to our kids without realising it then wonder why they have a meltdown. If your son only has as many things as will fill up 75% of the storage it will be a much easier task. I'm not saying throw stuff out, just be aware as the years go on and he accumulates more things he will need to move other things on or have some storage somewhere else.

My sister did the whole move from one thing to the next and my mother just let her and went around after her tidying up. Her life is chaos still at 32 years old. I often tell my mother that all she had to do was have that one argument with my sister when she was little and stick to it and maybe things would be different. I mention this because it's worth the tears now. It feels wrong but you did the right thing.