You say your DS's room is "miniature" - would it be possible to move some functions to the new room? His current room could still be good for sleeping, changing, and storing clothes, but if you made the "useless room" into more of a hangout spot he might get the benefit of the extra space without giving up his privacy (which he would have to do if you had to walk through his room to get to the carport and/or do laundry). Something like a comfy chair with a reading lamp near the window, a sofa (the kind you think is too ratty to keep in the house but your DH and your DS love - not that I know anyone with a sofa like that
), a bookshelf, a small portable table good for homework while sprawling on the sofa or a laptop/tablet if that becomes part of the daily routine, a folding table good for doing laundry on (dump the clean basket on the sofa and fold it from there), etc.
Key to making the space comfortable would be to have good lighting, no drafts of hot/cold air coming in from outside (i.e. a good sturdy door to the carport), and maybe some funky curtains or a funky blanket to go over the sofa to add some character
It would also allow you to move some of the less sightly elements from the more public parts of your house (living room/parlor/den/whatever) and separate your living areas into "the one we keep looking good so we can have company" and "the one we let get messy and don't care as much if it looks lived in."