There is no one telling you what your character can or cannot be. You don't necessarily need to flesh out his sexuality if it isn't relevant to the story. Just slip it in. Or give a ton of thought, if you want to. Sometimes, if you're stuck on developing a character, you can discover who they are by pantsing (Know that term? It means to write without a plan. Just dive right in with no barriers, throw him into some random situations, and see how he reacts. You'll probably pull together some characteristics of his. Pantsing might also help you figure out where you want your story to go.).
Being a female and finding other females attractive, I can't give you a straight man's perspective. But if you want your guy to be somewhat attracted to the female gender, aside from giving a girl some personality traits he likes, make him notice some physical things. I feel like the traits on a female that people tend to notice are:
- Lips (especially if this girl likes lipstick, right?)
- Boobs (sorry but it's true)
Yeah, there are more but you get the idea. You weren't born last night.
Depending on the personality and tendencies of your character, he would be attracted to different things. Honestly, this totally works for guys, too. If he views himself as strong and domineering, maybe make him notice the soft hair on a girl, or a guy's playful smile, etc, because they seem gentle. If he likes people who seem tough or 'above him' in some way, maybe a guy's muscles are what he notices, or how creepy and mysterious that dark-haired girl in the back corner is, but somehow in a good way.
Also, if his family was killed, depending on how, he will probably have some difficulties stemming from it. Maybe he doesn't trust easily. Maybe it's PTSD or anxiety. Maybe he has some crazy thirst for revenge. Maybe he's super confused about the whole thing. And if he's in a royal kingdom, is he entitled? Or does he have so much going for him that he feels guilty, and constantly gives to the less fortunate? Or does he himself not have a lot, in spite of that (I don't know how your fictional world works)? You can find many character traits and motivations from backstory, I've learned.
As for worrying about a primarily male cast, that's fine. Think of Lord of the Flies. There is not a single female in that story, and I love that book. But if your story doesn't center around a deserted island full of British schoolboys, it might be realistic to get a few women in there, or others in between. You just need to appeal to readers somehow and they shouldn't care.
Hope this was of at least some help. Good luck and have fun.