Hey @Nicko 1, welcome to Reachout! No problem about where you posted it, I've moved it for you to somewhere that's better suited 🙂
When your boyfriend expressed to you that he is attracted to girls, did he say that it was girls exclusively? It could be possible that he's having some issues working out his identity when it comes to being attracted to multiple genders, as it can be very tough (on both the person and their partner) when someone's sexuality come into question in their own mind.
There are people who, while they would otherwise identify with one sexuality, may come to be attracted to and in a relationship with someone who doesn't fall into that identity by chance, so it could be possible for him to identify as straight and still be happy to be in a relationship with you.
With either of these possibilities, what's important is that you guys are communicating openly with each other about how you're feeling. It sounds like you're feeling like you could use some additional support from him to navigate this new facet of your relationship and if he's going though a period where he's questioning his previous ideas about his sexual orientation he is probably in a space where he could use some extra support too. This can be easier said than done sometimes, especially in a long distance relationship, but it sounds like he's already making an effort to reassure you and you're doing your best to understand his situation, so I think you're well on your way.