I think you handled the situation with your youngest son well BUT here is an alternative angle from his POV that you might want to consider.
He is the youngest of 3, all his life he has had to stand in line to get attention (as I mum of three myself I can say this with conviction b/c no matter how hard you try to split yourself equally it never really happens in reality as the one who shouts loudest usually gets heard). Now suddenly he has one parent ALL TO HIMSELF. Just consider for one moment how good that would feel in ordinary circumstances never mind for a child whose mother essentially abandoned them. He's told you how he feels, as young as he is (which from memory is about the same age as my youngest) he is still old enough to make up his own mind and know what he wants. Respect his decision and if XW asks make it quite clear that it is his decision. Given him this time and eventually he will miss his mum, perhaps get a little jealous over what the other two are doing with her and make up his own mind to see her. That way there is no pressure for either you (and hence no guilt) and eventually everyone is a winner. Trust me it won't take long before he is itching to go over there to spend some time with her.
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15
