Let's not all pile on this person we haven't even met. Not everyone can or should be fully "out," even online.
I know you said it was on the internet, but a lot of times people show their IRL friends parts of their online lives, and being trans is a big secret for some people. Additionally, he might not want people who know him as female from other parts of the internet finding him being male elsewhere on the 'net and asking awkward questions. Finally, he might not have the confidence to mark himself as male online yet--he might think people would "see through" him and accuse him of just pretending to be a guy for whatever reason. (I had this fear for many years.)
You should be flattered that he confided in you about his gender identity. Try and respect the choices he makes about revealing it to others.
He may well be genderqueer, bigender, or non-binary (or confused,) but that's for him to tell you.
Maybe you could try letting him know that you're a good person to talk to, if he needs someone for trans-related advice (provided this appeals to you)?