My friend wont believe im gay?

me and my friends were out moutain biking and we got seperated because one of my friends had back problems but we never noticed because we were ahead in the trail. so it was me and my other friend at the bottom of the trail and we waited and waited and waited for our 3 other friends. so i took this as a opportunity to tell him. so i said "im gay by the way". after this he kept on saying "are you serious" and id say "yeah" and he would ask this sooooo many times and then he would say "are you messing" and then i just get nervous and say "yeah" in the happiest face i can even though i felt like crying. how can i just tell him and make him believee it because it kinda came as a surprise to him because idon'tt act feminine.