Visual Impairment -- Neurological/Synaptic?

  My grandson, age 5 1/2, lacks the ability to distinguish colors.  He can see very well in dim light, distinquish nearby objects and those at great distances, and exhibits normal optical acuity in all areas except color.  Apparently, he sees in a range from white to black (perhaps in the blue-gray range, as I understand is typical of canine sight), and the only detail which he cannot describe is color (hue, tint; he can distinguish value, and sees a wide range of grays).  Assuming that his retinas are formed normally (i.e., that he has the requisite configuration of rods and cones), and assuming that the area of his brain which interprets optical data is normal, is it possible that his optical synaptic "relays" are working improperly; that his brain has too little of an electrochemical enabling chemical (i.e., seratonin)?  Thank you.
=
There is a hereditary rare condition called achromatopsia, in which the rod/cone distribution is abnormal.
The more common x-linked hereditary colorblindness (in males only) is for red-green, but other colors are still distinguishable.
A condition in which serotonin were simply absent would be expected to lead to a whole lot more than just color discrimination symptoms, since 5HT is involved in so many systems in the brain.
I hope this helps. CCF MD mdf.