First of all, go through all the clothing items and decide which ones still fit or not, which ones can still be worn, and which ones he still plans to wear. Teenagers outgrow their clothes fast, so, if you know younger kids that can use these clothes, donate them. You can also donate them to shelters, to be used in disaster areas etc, instead of just throwing them away. Do throw away though the clothes that are ripped and cannot be repaired.
Once the old clothing is out, begin organizing the closet. Separate clothes and set them into categories: short-sleeve t-shirts, long-sleeve t-shirts, jeans, etc. Place them into drawers by category. You can also arrange them by season: summer clothes is one category, winter clothes a different category. Underwear and socks go into a separate drawer. Hang formal attires on poles, and keep them together. Don’t mix them with casual wear. Also place the school uniform on a pole, if necessary. It might be a good idea to organize the clothes into complete outfits, that way it is easier to spot them and get dressed, especially in the morning rush.
Put the shoes on the bottom of the closet; arrange them first into categories: sports shoes, casual, formal and school shoes. Place the accessories together, for example, try hanging the belts and the bags, if small enough, on hooks behind the closet doors. If that’s not possible, you may place them on the top shelf of the closet. Check if the closet has enough shelves, it might be a good idea to install some more, or to expand the closet, as kids grow up.
Also, you can place a basket for dirty laundry in the closet. It might come in handy for the teenager to throw the dirty clothes there, rather than throwing them on the floor. Also, put an air freshener in the closet.