Is there any sexual orientation he could claim to have that you wouldn't doubt? Like, if he mentioned being straight, would that have surprised you, because you think he should be undecided?

Although I'm asexual myself, I'd almost consider pansexual the "default" sexual orientation--it's what you get when you do feel sexual attraction but don't have any particular genders excluded from this attraction as a rule. So if a person actually would need to experiment to know their attraction level to different groups, and they haven't done that experimentation yet, what reason is there to think they aren't pansexual?

Besides, it's a pretty blurry distinction between sex/gender-based biological attraction and all the other reasons a person might desire, enjoy, consider, dislike, or refuse having sex with any other particular person. Half the time the difference between someone who is heterosexual/homosexual versus someone who is pansexual might just be that the pansexual person is not committed to [insert non-preferred gender] being a dealbreaker for potential relationships. (And how many people of a given gender do you have to evaluate before you can be confident you'll never be attracted to anyone of that gender? I have to admit, I have trouble wrapping my brain around monosexuality.)