Is there a way to find out if my fiance is a legal citizen?

My fiance was brought to the United stated in 2001 by his mother from Canada. He was enrolled in school, graduated, and has even gone to court before. However, his family is still saying he's illegal. His step dad has no idea if e is or not, and his mother has schizophrenia/paranoia. She won't tell us anything except if it has something to do with her "stalkers". We're wanting to get married, and set our ground roots, but if he's illegal, we're going to have to go through a lot of hassle and a lot of money to get him citizenship. He has his birth certificate and his SI number (from Canada. He has his high school diploma and his old school I.D cards. Is there any way we can figure out if he is illegal, or if it's a hoax?