My brother in law who just turned 18 just got caught crossing the border and is being held. He has no family left in Guatemala after my mother in law passed away. What can I do?

Were is he being held? What country are you in?
My guess contact friends back in his home country to make arrangements for him to return to.

It will depend on what his basis is for staying in the US or what relief he has from deportation. The fact that he doesn't have any family in his home country is not a basis for a lawful stay in the US.

Consult with an experienced immigration attorney. Good luck.