What should I do? Tell the truth? Or to just let him think I like him?

He thought I like him, but I really don't. I don't want to upset him. What should I do? Tell him the truth? Or to just let him think I like him? He likes me, though. He thought I like him, because one of my friends told him so.

Flashback: "Hey, Kyle! Shinta likes you!" my friend, Mika, said. "What?! She likes me, too? I've liked her for a long time!" he said. I just ignored them because I think it was just a joke-or so I thought.

"Hey, Shinta. Since you like me, too, may we go out?" I thought it was a joke so I just laughed at him. He became serious, so I stopped laughing. "I'm dead serious, Shinta. I like you. For almost 2 years, I think?" I'm shocked! I didn't expect it to be true. I just smiled and walked away, leaving him with a sad look on his face.

End of flashback

So, I thought you might help me, that's why I'm posting this. Thanks in advance! God bless!