Let him CIO and don't go in as it usually escalates them ..... Put him to bed shut the door and walk away go back when nap time is over and it won't take long for him to figure it out ...

When a child here says mine when they are playing with a toy that another child is trying to snatch I say actually its mine and in 5 minutes you have to let someone else have a turn if they are hoarding toys or are saying mine to a toy they are not playing with then they get told it's not theirs and xxxx is allowed to have a turn.