About 4 months ago I started having feelings for another boy in my year. Literally, I know you'll all think like oh he's 15 he doesn't know what love is... But believe me right, I would do anything to date him for a minute. Abyway, point is I love him, we are close friends, every time I try and tell him he thinks I am joking...so can't really...uh... He is bi, came out to me last month and I was the first person he told. How can I convince him I'm not joking? Never really understood what love felt like before now... But man it hurts not being with me yanno?
You could start off the conversation with something like, "Hey I have something I really need to tell you and it's serious" or just like sit him down so he knows that it's a serious conversation and then tell him that you're not joking when you say that you love him and you really do.
Why does he think you're joking in the first place? Be firmer. Tell him that it makes you upset that he thinks you're joking when really, you're not. Perhaps he is afraid that it might strain your friendship if the relationship doesn't go quite right. Either way you two should have a real conversation about it.