Easiest way to obtain US citizenship?

My husband is a Guatemalan citizen & has been a U.S. permanent resident for over 30 years (he's 35 years old). He is estranged from his parents but it is alleged they both became US citizens prior to him turning 18. I was born an American citizen and we have been married 7 years. What is the easiest way for him to obtain US citizenship? He has not been to Guatemala since he was a child.