Inspired by his request of a tree house, we put his room on the upper floor at the nearest spot with the tree. It contains two levels: Level 1 - Bedroom, Level 2 - Upper terrace. He can climb the iron stairs to the upper terrace and hangout with his friends there.
I made the head bed with one unused glass door. Painted it with geometrical white stripes resemblance the tree branches.
Collect some of his belongings, choose the most decorative, got this aesthetic values which fit with the room concept in particular. Of course at first we have to ask him his favorite color. Yes, BLUE it is.