I have written and deletes three replies to your post now, each reply I have deleted because I feel I am being too brutal and not supportive enough.
I shall try again.
You need to be the parent here, not a pushover.
Your baby needs a proper bedtime now. Being up until late with Mum and Dad is fine for continentals who have siestas, but this doesn't appear to be the lifestyle you are after (given that you do attempt to put him to bed at a reasonable time).
You've tried everything and "nothing works". I don't believe this. I believe you have tried everything and I understand that you must be very, very frustrated because these things take such a lot of perseverance and effort.
But if you and your DH make a decision and stick to it - not giving up and bringing him downstairs, then it will work. It doesn't really matter which option you choose - you have tried them all so must know them already. Shush pat, gradual withdrawal, no cry, controlled crying, Cry it out... and so on.
You just need to pick one and stick with it until it works. Giving up a technique and trying something completely different next time is very confusing and must leave your son spinning not sure what he is supposed to be doing. Plus he always ends up getting what he wants anyway, so he just needs to keep on crying and (in his mind) all will be well.
You need to un-teach him this so that he realises all will be well and he will be better, less tired, if he sleeps in his cot.
It will be hard work.
He will get very upset and angry with you.
It may take a long time (some methods mentioned will take longer than the 3 weeks you have).
But if you pick one method, are consistent and stick with it - it will work.
Good luck