WordNet (r) 3.0 (2006):

boyfriend
    n 1: a man who is the lover of a girl or young woman; "if I'd
         known he was her boyfriend I wouldn't have asked" [syn:
         boyfriend, fellow, beau, swain, young man]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "boyfriend":
   Casanova, Don Juan, Lothario, Romeo, admirer, amoroso, beau,
   beloved, caballero, cavalier, cavaliere servente, crush, esquire,
   fancy man, fellow, fiance, flame, gallant, gentleman friend,
   gigolo, heartthrob, inamorato, lady-killer, love-maker, lover, man,
   master, necker, old man, paramour, petter, philanderer, seducer,
   sheik, squire, steady, sugar daddy, swain, sweetheart, truelove,
   young man