What to do when your cousin says he likes you?

OK, I really need help, not so long ago, my cousin expressed his feelings for me. After a few days he told me it was all a joke but then he tells me his serious about and that he wants to go out with me. I said no but he keeps asking and I don't know what to tell him. I'm not sure if this is all a joke or not, hopefully it is but what should I do? Should I believe him now that he said his serious? Or just forget about it? And if he is serious, what do I have to do?
What to do when your cousin says he likes you?
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