State / Country: California / Australia

My brother in law who is a Marine on active duty married a woman in the United States who is from Australia. She recently went to Australia and she is now refusing to come back. The child was born in Australia (wife demanded the child be born in Australia and become a citizen of Australia) and the child still is not a citizen of the United States (the mother in law has convinced her daughter that the child should not a become a citizen of the United States). Is there anything my brother in law can do to get his son back? To add to the situation they have another on the way.

I hope I posted this in the right forum.