Question

what might be the cause of onset of social fobia in a 19 year old boy?

Answer 1

http://en.wikipedia.org/wiki/Social_anxiety_disorder

Answer 2

Has the boy been diagnosed with a social phobia? That diagnosis is typically not given unless the symptoms occur over an extended period of time, usually 6 months or more.

If this was not part of his behavior before, and the onset was rather sudden, look for a recent traumatic social interaction as a potential cause. If none can be found, have him see a physician to rule out potential physical causes.

I hope this helps. Thanks for asking.