Im assuming you have parenting orders. If so.
There is family case law here suggesting the parent needs to know where the kids live with the other parent. A judge would probably look upon the parent who didn't tell in a bad light. Obviously risk, family violence, etc, is an exception.
I have shared care and my ex moved recently and she didn't want to tell me where she resided. Although my orders specified some handover to be at the other parents residence (ie, I need to know the address), I was about to file a motion on the presumption I, as the father and having shared parental responsibility, should know. The day before I said I was filing it she gave me the address.
For 2 days however, I'm with Sammy on this one. If he starts getting more custody and overnight care and still won't tell you then I'm sure you have a case to ask the judge to force him to disclose.