Does he wake every night of the week and at his Dads house when he is there,have you asked him why he is waking, is he dreaming ,what do you do when he wakes, just tell him go back to sleep, do not go into his room or you are feeding into the issue, tell him goodnight then leave him, if he calls out respond then let him go back to sleep. Make sure he is happy at dads house, are there other children there ,?
There is homeopathic medicine to help with sleeping - I admit my daughter has trouble going to sleep not waking up in the night. Is it night mares ? we do go thru stages as a kids in having night mares - I still remember mine and I'm 39 (ouch !) It is comfort then and reasurance that he is looking for and perhaps he doesn't get that from his Dad ! - Men function differently with women and he maybe needs this from you and doesn't get it from Dad (pretty normal I'd say !) good luck