My son is married to a girl from mexico who is unhappy with him, could he get custody of their son?

Are they together or separated? If they are separated he should move for an order for joint legal and joint physical custody NOW, as well as an order restraining Wife from leaving California with the child.  There are also child abduction orders that can be issued further protecting the father from becoming embroiled in a lengthy international custody dispute.  Is the father on the birth certificate? Does the child have a passport? These are other considerations that need to be addressed. I would suggest that he meet with an attorney to get better guidance going forward.