Hmmm, which common house plan could this be...

(I grew up in SW Michigan.)

I assume your upstairs is a T shape, with gable ends at each tip of the T, and a (nearly) flat roofed bump out along the top of the T. The big room is at the base of the T, the itty bitty room is the left half of the top of the T, and DS's future room is the right half. Further, the base of the T has the tallest gable and points to the street, and the bump out overlooks the back yard. Yes, no?

I suspect the bump out might have been just a gable end originally. Also, you might be able to have a normal ceiling height in the hall.

How about moving the door to DS's bedroom into the hall, thus combining the two rooms. I think the door could swing into the bedroom against a former hall wall and not block anything. Then configure the itty bitty room as a closet and computer nook for DS.

I see no point in definishing the itty bitty room. Livable space would be better than totally unfinished attic.

You probably want some sort of proper flooring upstairs. Carpet is probably the cheapest option. It'll also feel warm on cold Michigan mornings. A hard surface upstairs might be very noisy downstairs.