My name is ***** ***** I'd be happy to answer your questions today. I'm sorry to hear that this is happening.
The father can go to court and file for divorce or legal separation plus temporary custody. Utah allows divorce on the grounds of irreconcilable differences. Alternatively, he could file for legal separation. A person is allowed to file a Motion for Temporary Custody at the same time as the petition to get that ball rolling as soon as possible. If there is any reason to believe the child is in danger, the father can file an EMERGENCY petition. These petitions will order the mother to produce the child, and if she doesn't, she can go to jail.
I realize that this may seem a bit extreme, but there are no laws governing how parents must interact or treat each other during the marriage(absent, of course, violence). To get a court of law to step in and tell the mother that she needs to let the father see the child, there needs to be an action for divorce or legal separation pending.
Another option is to send the mother a letter, detailing what action will occur if she doesn't produce the child immediately.
If at any point the father knows which relative the child is with, he could call the police and go over there with them. A third party relative has no rights to a child without the parents consent, and since mother and father legally have EQUAL rights to the child, they cannot keep the child over the father's objection. So, in that scenario, it may be possible to find a police officer to help - they're essentially guilty of kidnapping
at that point.
It would probably best to have a local attorney who could help in this scenario, at least until temporary custody is established. But the courts do have some information online you can use to represent yourself, including forms:
It's important that you are 100% satisfied with my courtesy and professionalism. Thank you.