Hello, Katy!
Information is a good thing to have. There is a lot of wiggle room when it comes to gender, and it can be very difficult for someone who has questions about it to ask (or explore) them. It can be extremely difficult for them to ask for or accept help. My mother came right out and asked me if I wanted to be a girl when I was six, and I could not admit that I did because I knew it would not be safe to. If there are people in his life who he feels would not understand and accept hearing that he wanted to be a girl or felt like he was supposed to be a girl, it will be difficult to get him to open up.
So, the first thing to ask is, would it be safe for him to open up?