The Three Investigator series might also appeal to your son. As a preteen girl I read a LOT of the Nancy Drew books, and later also enjoyed the 3 Investigators. I never read the Hardy Boys books tho'...

As a kid in the 70's I was a voracious reader. After reading all of the Hardy Boys books at the time in the library, I moved on to Nancy Drew. Then to Encyclopedia Brown. By the time I got to high school, I had read all the books in all the series at least twice and had read favorites like 4 times. I loved Nancy, Bess and George and thought the boyfriend, Ned was a dumb twit. I couldn't believe someone as smart as Nancy would hang with someone as dumb as Ned.

I have no idea why a boy who loves detective stories would not like them. Personal tastes may vary, but there is nothing inherent in the books that would make boys dislike them (for example, they are not androgynist).

I loved Nancy Drew and a little later Hardy Boys as a kid and my younger brother read whatever I did. Thankfully we did not know then there were "girl" and "boy" books and neither did our parents! OP let your kids explore and encourage their love of reading (unless something totally age inappropriate)