What do I do?

So I'm 26 and I'm ftm, I have a male best friend who's bisexual I have for a couple of years now, we like rping and playing games and note he's my online best friend so we live in different places far from eachother, and I think he thinks I'm a cis guy, idk if I'm technically lying to him or what, he's I think 18, and he promised no matter what he won't ever leave me and he told me a couple weeks ago he like likes me and that I'm the first "dude" he ever liked so he told me he's gay for me, ever since that day my stomach started feeling weird like a bad weird like if I'm guilty and I don't like this feeling, I want to tell him but I'm scared of our friendship breaking up, what do I do?