Should we steal 2-feet from our son's bedroom to make our master closet more useable?

We are undertaking a major kitchen renovation in our house in Toronto, Canada, but will also be making a few changes upstairs, primarily updating our master bathroom. The only structural change we're considering upstairs is to take 2-feet from my son's bedroom to widen our existing master walk-in closet. Since we are currently living out of the country, it makes sense to have any "messy" work completed all at the same time, since the house will be vacant during the reno and we won't have to live through the mess. The entire upper level of our house will be re-carpeted before we move home, so this seems like the time to move a wall, if we're ever going to do it.

Our existing walk-in closet is very long, but quite narrow, so it can only be fitted in an L-shape. If we widen it by 2-feet, we'll be able to comfortably fit it out in a U-shape with custom hanging, shelves, drawers etc. Currently, we have one standard builder clothing rail with a shelf above it (very ineffective use of the space!)

Here's the current layout of our upper level. My son's bedroom is, unfortunately, already the smallest bedroom in our house at 13'8" x 11'0, so not ideal as the bedroom to be making even smaller, but that's just the way it is:

My only concern is whether or not we would be making a mistake to make my son's room even smaller. We're not particularly concerned about re-sale as this could be called our "forever house" (as much as I believe in such things, since I've lived overseas a few times in my life!) My son will be 12 years old by the time we move back into our house in Canada, so he'll be living in that room through his teenage years. We have a good-sized finished walk-out basement in our house and will be installing a pool in the backyard before we move home, so I'm guessing the basement and backyard will be our son's primary hangout spaces with his friends (and not so much his bedroom). So (as a parent who doesn't yet have teens), I'm guessing his room will be primarily used for sleeping, clothing storage, homework and having a space to go when he wants to be alone?

I've mocked-up a furniture plan for his room that would give him:

• reach-in closet with double doors (existing) (the drawings show sliding doors, but they are double doors that open into the room, not sliding doors)
• double bed with tall bookcases as "headboard", to allow for storage and a substitute for nightstands, since not enough space for nightstands
• built-in wall-to-wall dresser/desk under window with floating wall shelves for display

As long as his room is good enough for a teen, my strong preference is to make our walk-in closet as functional as possible. Are there any compelling reasons we should NOT proceed with shrinking the size of our son's room? Or does it seem like it will be functional enough (and comfortable enough) for him at the new proposed size?

Thanks!