Is my father-in-law already a U.S. citizen?

My father-in-law was born in Canada in 1950 to a mother who was a US citizen through birth and a father who became a naturalized US citizen (Canadian born) in 1940, I believe. They (his parents) lived very close to the US/Canada border and lived on both sides of the border before he was born which was, eventually, on the Canadian side. He came to the US side with his sister when we was about 14 and has a social security number (he has since lost his card, however). With the new laws for crossing the border (needing 2 forms of ID recently and now needing a passport or passport card) to visit some family that he still has on that side, he's afraid to go because he thinks he may not be let back in. Can he just get a passport/passport card, without having to go through another process, as long as he has his mother's birth certificate, his father's naturalization number/card, and their marriage liscense, and his birth certificate?

He can get it with his birth cert if he was born on the US side, otherwise no, he likely will have to go through some process to verify his citizenship.