Okay, then because he wasn't married to Mom, he had to have signed paternity papers. This is good news for him, because now he's the legal father and even without a custody order Mom doesn't get to just up and leave and conceal the child from him.
He needs to get himself to an attorney - as in the next hour or two - and file for a temporary order pending a final determination of custody. His attorney can guide him on whether this is or isn't enough of an emergency to file an ex parte order (usually limited to imminent risk of harm, but can also pertain to a parent who takes off and there's a decent chance the child will be removed from the state).

