My baby won’t sleep unless with a blanket😫

I put my full sized throw blanket in his play pen once and ever since then he won’t go to sleep without it. When I put it in he will literally fall right into it and fall sleep.

He is 9 months old and after he falls asleep I move him into my bed at night, but I let him sleep with it when he takes naps. but I want to start letting him sleep by himself through the night. I always am supervising when he’s in there but how should I steer him away from wanting it? He literally won’t sleep without it