Question for moms that rear face and extended rear face their child's car seats !

My son is 5.5 years old and we are still rear facing, as he is super small and way down in the growth chart for height and weight. His pediatrician recommended we stay rear facing and I of course have also done my research on that too.

Anyway, he's always been in the center seat since he was an infant , but as he gets bigger it's kind of a struggle for him getting up in the middle. Is it safe for me to move him to the seat behind the driver's seat (still rear facing) or is it best for me to keep him in that middle spot? We don't have other kids or anyone sitting back there.

Thank you !