with that much lean, as mentioned above replace the seat back stops,What is required to fix this bench seat from leaning back?
View attachment 397761
and as mentioned above if that does not correct the lean disassembly is needed to inspect the seat back frame then you may find the frame has failed and needs to be repaired,
to inspect the "arm" simply remove the side upholstered piece held by 2 Phillips head screws,
the lean on my front seat was due to a broken original weld marked in the circle except on the drivers side