Maybe you could tell him that he will need to go back into the cot, because a big boy like him should be able to go to bed and stay in bed (I know he should understand that rationale) but if he can't stay in bed he will go back into a cot and it upsets you and Dad when you don't stay in your bed etc. Other than that I really don't know what else you can do, because I know you've tried almost everything already