My boyfriends back has been bothering him. he says it feels like carrying heavy grocery bags when they hurt your hand. what could this be ?

Lumbar strain: There are several possibilities & thus he should see his primary care physician for starters who can do a physical exam & consider all pain sources.The most likely cause is lumbar strain or muscle spasm.This usually responds to a muscle relaxant & PT.Other causes could be disc disease, spondylolisthesis,kidney or sciatica,If initial treatment does not work,he will be referred to a spine specialist