Can a man consider himself bisexual if he is in a relationship with a girl and has never been with a man?

Yes, he can. Your orientation is determined by feelings, not experience. Bisexuality is an identity, not a "track record." Being in a relationship with a woman does not make you "simply heterosexual," just as being in a relationship with a man does not make you "simply gay." Imposter syndrome is common among bi people, but remember: you don't have to prove anything to anyone. If doubts persist, consider support from a psychologist familiar with LGBTQ topics.