tough to tell. if the vision/eye tests and visual fields were normal, it is unlikely that he has anything serious wrong with his eye. a brain tumor that affected his vision would almost certainly show up on a visial field test. its usually very sensitive to that sort of thing.
if the spot is dull black or grey, we're probably talking about a floater, even if the doc didnt see anything, and even if it doesnt sound like a floater to you based on your son's description. if the spot is BRIGHT and always present, that soulds either entopic or neurological, and you'll have a very, very difficult time finding someone to run a bunch of expensive tests (mri, etc) to see what it is. everyone will likely tell you to wait it out and see what happens.
we could be talking about a migraine aura or ocular migraine (even if your son does not have head/eye pain) here, which would be difficult to confirm. but when someone says a spot "doesnt go away", that makes migraine unlikely as well.
my guess is still floater, especially if the spot is grey or black. it makes the most sense. a spot in your vision that follows your vision (you look up, it moves up, etc) is almost always a floater. a spot in your vision that does not move (you look up, it still "looks" straight ahead) is pretty much impossible.
the serious stuff like retinal detachment, hole, eye tumor, bleeding, etc has assumedly already been ruled out with the dilated eye exam...