DH suggested it last night, think it could be a runner. Not madly keen on Frank as a nn, though - maybe Frankie? Or not? Fran is a bit girly, maybe...
Can't quite imagine it as a baby name, but imagine he would grow into it as a boy.
Other possibles are Jonah (but have an Isaac already, so poss a bit much), Laurence (Laurie), Alexander and Hal (poss to be called Henry but known as Hal).