Don't know how old your son is, but it could serve different purposes as he grows. It would be a nice reading spot--rather than building in a bookcase you could do shallow wall mounted book racks (Ikea carries them). As he grows older it would make a nice desk/study area. And kids always love little nooks and crannies to play in, so a little bookcase could hold his favorite toys. I would consider having a wall mounted fixture wired now, though, as you need lighting for reading/studying. Love your windows!

How about a small chest of drawers (height of the windows) on the back wall, with a mirror on the back wall above it? Or a small window seat with padded top and lift up lid for toy storage? How about a lamp in that nook with an easy chair for reading? Depends on how the rest of the room is laid out...maybe just put a full length mirror there and ignore the nook.