nursing while standing up

help!

my 2nd baby is a preemie. he’s 5 months old c.a 3 months and he’s ebf. he recently prefers to nurse only when i stand up. with my firstborn, she liked to nurse lying down and we both get to relax that way. but not with my 2nd baby, he refuses to nurse while i’m laying down and sitting down. sometimes it’s a success when he’s drowsy but he never wants to nurse lying down while he’s awake. he’s getting big and i’m starting to have back pains nursing him while standing. could there be any underlying issues with him that he hates nursing lying down? any advice?