Stress, anxiety, diet...I am sure numerous other things. I wouldn't be surprised if it is more the first two. If your son is consistent in his care, then your g/s knows what to expect when he is with dad. But then going to mom's house, that is a change in itself, and if things are different, he is going to respond differently (including natural body functions!). I would talk to your g/s dr about the Miralax (and make mom go along to the appt so she hears it too!). If he poops regularly other times, it might not be necessary. :-/ Sounds like a sticky, not fun situation...good luck!