Your youngest son may just be too young to deal iwth this problem... It may be a bit over his head... THat's why I suggested just your oldest son and you daughter... you three sit down and tell him that he is not getting a get out of marriage free card and there WILL be consequences for him abandoning the family... and USE that WORD...
As long as they or you make it easy for him to walk away, he's going to lean that direction...
