If not the occular migraines, perhaps his optic nerve is being pressured by the difference sitting versus lying down? My coworker got occular migraines and her hormones (too little estrogen) were the main cause -- hormones cause regular migraines as well. One thing I was also told was diuretics helped decrease migraines.
Might help to keep a journal of not only when he sees them, what he's doing, have him draw if he can, but also keep a daily journal on his activities, eating, eliminating and such for a few weeks.
If the doctors are stumped, pump them for other referrals until you get help. I had client that had multiple retina detatchments, and he needed to see 3 doctors before he started getting the correct help for his trouble. I had to go to two OBs before I had the "right" fit for something as simple as childbirth!:)
You have every right to continue investigating until you find a solution!:) You are your son's advocate and all you do will help him and you both!
Let us know and my prayers with you!