My thinking is to get Son and Daughter together and just hit him head on one night :
All three of you sit and say to him OK, this is your choice
a.Work on the marriage and make a 100% commitment to put forth the effort - no exit activity and no exit talk whatsoever b. Leave now and not be a part of this family any further
That's a rough choice, but with all three of you staring him down with a rough exit like that I would like to think he would cave ...
