Demiboy
Partly a boy or man, and partly something else.
A demiboy feels a partial connection to being a boy or a man. The connection is genuine; it just doesn't account for all of their gender. The remainder might be another gender, or none.
As with demigirl, it doesn't follow from assignment at birth. Someone AMAB may be a demiboy because manhood only ever fit in part. Someone AFAB may be one because it fits in part and that part is real.
The word is precise in a way that's hard to reach otherwise — most gender language forces a whole answer, and this one allows a fraction.