Aww giant hun it's not easy is it, i remember when we used to bring Ava into our bed while i was PG cos it was easy and i needed the sleep, it lasted forever and i thought she would NEVER stop.
In the end i persevered with re-settling her in her own room as i knew i couldn't have her coming in and waking up Z when she was born, it took a good couple of weeks to get her back to sleeping properly

Is there any way you could maybe involve him in re-decorating his room to make it a little more comfortable, some exciting new doona cover etc?
Does he have a soft toy that he can associate with bed?

Ava is on a mattress on the floor in the new house (beds on order) and she is sleeping heaps better than in her cot, she loves her big girls doona and pillow.
We have the door closed tight and she can't open it as she's too short, maybe you could put a baby gate on the door.. as much as it will be a PITA you would have to get up and maybe persevere with re-settling him.. even if it means falling asleep in his bed with him for a bit?

I'm sure someone has some great advice, you've probably tried nearly everything as it is.

GL