Sorry to hear about your son's recent diagnosis, but glad the MRI was normal. Yearly checkups by an optician (someone who helps with a fitting for glasses) rather than an ophthalmologist (who is an MD and does more detailed funduscopic examinations) would not necessarily pick up papilledema. Another possibility is that he just developed the condition since the last examination. Papilledema is indeed swelling of the optic nerves and indicates increased pressure in the brain. Bathing your brain and spinal cord is a fluid called cerebrospinal fluid or CSF. A normal pressure of this fluid is usually 10-20mmH20. There are many things that can cause increased pressure and thus papilledema such as a brain tumor, head trauma, infection, and blockage of the normal flow of the CSF. Since your son's MRI is normal, there is a possibility he may have a condition called pseudotumor cerebrii which means increased pressure with a normal brain. This disease has no known direct cause, but is associated with obesity, fad diets with too much vitamin A, medications such as tetracycline and other agents used for acne, as well as other things too numerous to mention here. A spinal tap will measure the pressure of the CSF (MRI can't do this) and help the docs make the correct diagnosis and form the right treatment plans.
It;s important to know whether or not he actually has optic neuropathy (a problem with the optic nerve that has many causes including congenital)or just papilledema which is really a sign of increased pressure rather than something you're born with. Talk to your primary care doctor about getting a formal neurology and neuro-ophthalmology consult. If there is no neuro-ophtho nearby, then get a good ophthalmologist to take a look and do formal visual field testing. The long term consequences of papilledema is blindness if left untreated and it's important to document what he has now and down the line. Good luck