What should I say to my cousin who used to like me?

Okay so I have this 12 year old cousin who is about to turn 13 in August. I’m 15. Let’s call him Alex. Alex is my favorite cousin, he’s funny but can be crazy. I obviously don’t like him other than as a cousin.

Whenever I see him and we are alone he says how he used to like his cousins before he knew it was wrong. He would tell me to try and guess who, and I knew it was me but I didn’t want to say that because it’s weird. He would ask me until I finally told him. He ALWAYS asks me even though he knows I know.

Also, we used to play fight. He would always wanna do it in the dark and he would throw me down on the bed and try to wrestle with me. He’s always asking me if I would date him if we weren’t cousins and I’m like no, you’re too young plus idk it’s weird to think about. What can I say to make him stop this weird stuff without being mean? Will he grow out of it?