You may be surprised to find that some of your friends will have already figured things out anyway, at least they may have if they're people that you've seen or spoken to during the year. They seem to have a way of knowing that something isn't right with you whatever you say to them.
I'd just sign the cards from you and your son, maybe add the line about 2014 being a better year than 2013 was if they're closer friends/family and just leave it at that. If they want to know, they'll ask won't they.
Both 50 S14 M 16 yrs (his 3rd; my 1st)
ILYBINILWY - 24 Dec 2012 H moved out - 27 Jun 2013 Legally separated - 6 Sep 2013 Closing the door and changing the locks