he just wont get it, help me out here!

my friends nephew has a crush on me, hes 2 years younger than me, and just recently told me he likes me, we’ve been fiends for a while (ok only like 3 months) and he always talks to me and stuff, which im fine with, but he always calls me aheratbreaker (as a joke, but being serious at the same time). he just doesnt get it, hes immature, and hes in the seventh grade, I dont like him, he doesnt get what “liking people” is about, he thinks he can change himself so id like him, he asked me “what dont you like about me” and he also askes “well what kind of specs [do you like in a guy]” which I know hes gunna try to prove himself, or try and act like someone I may like, he also doesnt get that I have a crush he knows, and hes a bit miffed (and what makes it even more interesting is that my crush is actually his uncles best friend, whoich you know we have our little group and I like why freidn, and my other friends nephew likes me..just summerizing), and my crush like me back, so when hes asking me “what if your crush died?” he has no idea that hes talking about our friend..I dont know how to get him to just forget it, I already told him I dont like himin that way, and that we can still be friends, but he stil talks about it, how can I get him to understand with out being extremely harsh?