I am sorry to hear this. Hang in there. I know, easier said than done.
Has your husband actually spent any time with the OW yet? Or has all of the interaction been online? It seems weird for your H to throw away a family for a relationship that may not really be real. I think reality will bite him someday and he will regret it.
The main thing to keep in mind is to focus on yourself and your son.
Me: 43 W: 37 Together: 18 M: 15 D: 8 yrs old ILYBNILWY: March 2011 She Filed for D: August 2011 She moved out: Sept 1, 2011 Reconciled: May 2012 Divorce Case dropped: July 2012