S.M.
Does he have all his teeth? One thing I've noticed is when our wonderful kids get weird...it is always about the teeth. I believe they get "2 year molars?"
You could look into that, and if that is the case I would wait it out and give him the comfort he needs. Always start him out in his room at night and let him come in. But then once the pain passes do the technique the other mom spoke of: walking them back into their room. It worked on Supernanny.
By showing compassion but being firm your son will learn.
Best of luck.