Googling "Bell's Palsy" came up with this link: http://www.bellspalsy.ws/
It has nothing to do with cold, but is caused by trauma to the 7th cranial nerve, and is not usually permanent. The National Institutes of Health http://www.ninds.nih.gov/disorders/bells/bells.htm says that most scientists think a viral infection is the usual cause. I hope he gets better soon.