D.P.
Oh no! How embarrassing! I would share with him O. of YOUR embarrassing moments from childhood and have your husband do the same. Try to reinforce that it is not the end of the world and that in a few days, the kids will have something else on which to focus. Poor kid!
How would he react to making it an inside family joke? It might lighten the mood if he''s that kind of kid.
(On a side note--I wonder why this happened? Did he just wait too long? I have a friend with a 7 yo son who was just diagnosed with encopresis. her ped said it was very common around that age. Just a thought.)