Has he been evaluated for narcolepsy? Narcolepsy commonly manifests as what looks like Bells Palsy, but is actually called cataplexy (if that's what your son's problem is).
I went undiagnosed for about 15 years, until I finally saw a sleep specialist and she nailed the diagnosis right away. The vast majority of doctors have absolutely no idea what it is or what it looks like, and no routine lab tests, MRI, CT scan, x-ray or anything of that nature will detect it. 75% of people with narcolepsy never get diagnosed in their lifetime.
It doesn't look the way you think it looks. Common misdiagnoses in adults are depression, epilepsy, and migraines. I struggled for years to find an answer, and now I finally have one. It can be caused by viruses or certain vaccines (for example, H1N1 vaccine was notorious for causing a swath of narcolepsy cases in children).
I believe I am genetically predisposed and contracted narcolepsy after either my MMR or my T/D vaccine when I was a young teenager. I was misdiagnosed with ADD, and then about a decade and a half later, a few particularly bad sinus infections, I ended up with narcolepsy that actually included cataplexy (random muscle weakness that can either be isolated or widespread). It manifested first in my face, as a sort of Bells Palsy, but when I paid closer attention I realized it happened to my other muscles, though it was very hard to notice.
It took me awhile to figure out what my real problem was and it was years before the cataplexy set in. Have him evaluated, maybe?
This is a defunct discussion from 2011