My 14 year old just told me he's bisexual, so I did what I always do and had a google for things I might need to know as the parent of a bisexual teenager.
All the advice to parents was aimed at people who have a problem dealing with it, are worried about family members being prejudiced, a bit socially ashamed. I'm aghast really: we are not worried in the slightest and all I wanted was a list of things which might be issues in future that I hadn't thought about!
So: is there anything I need to know, as a person who's lived a very straight life, which may be something my son needs to deal with in the years to come? I am already checking out his school's policy on homophobic language and bullying. What else? Or nothing at all, I am overthinking?
I don't think at 14 that he can possibly be certain about his sexuality. I have a 16yr old dd so I'm coming at this from some experience. A good friend of my dd's wrote a note when he was 14 saying he was gay and he left it in the table when his mum was out and went to school. Fast forward a couple of years and he is clearly heterosexual. I think younger teenagers can confuse curiosity with an actual definite leaning one way or another.
During my teenaged years 50% of my peers went through a bisexual phase. The vast majority ended up realising that they were either straight or gay afterall. I don't think that you need to be worrying about the long term just yet.
My 17 year old dd is bisexual. She tells me that there is quite a lot of prejudice toward bisexuals, from both straight and LGBT communities. Bisexual people can be thought to be hiding that they are actually gay, attention seeking, messing around or uncommitted (she'd be pretty pissed off with the 'it's just a phase' comments here) or promiscuous. dd says that boys are particularly likely to be told that really they are gay.
dd knew that she found both boys/men and girls/women attractive when she was quite young, I see no reason why the OP's ds shouldn't know who he fancies at 14. The degree of preference may change as he gets older, and it might be that he only has serious relationships with people of one sex or the other but that doesn't mean he is necessarily bound to be straight or gay and not bisexual.