one | the sleeping area
What kind of bed does your son need? My son is tall and when he moved into this room, we knew the bunk bed he had been using needed to go to his little brother. He's still too tall for his twin bed, but he makes it work. What about bedding and pillows? Do you have enough blankets to keep him warm as well as a storage solution for extra blankets for when it's too hot? How about a bedside table for an alarm clock, light and any other important items?
two | storage (clothes + all the "stuff")
How are you going to store the clothes? What about dirty clothes? Hang up clothes and folded clothes need easy access so that your son can easily put his things away...have a kid who constantly misses the dirty clothes? Ask them what would make things easier for them and make them a part of finding a solution. For every item of furniture that we brought into my son's room, we made sure that it had a storage function as well...he is a kid who likes to keep things, and because I knew that going in we were able to find storage pieces to help conceal some of the "stuff". There are bookshelves and storage shelves and cubbies for him to organize his treasures.
three | study
Does your son study and do homework in his room? Do you need to accommodate a computer? What about chargers and other devices? There are some great items out there that can charge several devices at once and some great cord management items, a quick google search or trip to your local Target or Ikea will give you lots of ideas. Does your son love art and have a need for the storage and/or display of these items?
four | display
What does your son love? This room is his room that he will remember when he's grown and that will be his sanctuary as a teen. You want it to be a place that he loves enough to want to keep clean and the he feels comfortable in. Is he a music lover? A sports lover? What about comic books or super heroes? What can you incorporate into his room to really let his personality shine through? Think about shelving, art, trophies, posters, etc.
five | the finishing touches
The finishing touches are things like the color scheme of the room and how you tie all of the little pieces from the above parts together. What color paint? What about the furniture...all wood, the white Ikea look, metal or industrial? What about window treatments? Flooring and lighting? Make sure that you have the right amount of lighting in your son's room for general overall lighting all the way to task lighting for desk and bedside needs.