My son's lower back is sore by his kidseys.one minute his fine nexts it hurts to walk.could it just be a muscle.?

Probably-: If he is without fever, abdominal pain, nausea, vomiting, urinary issues, numbness/tingling or weakness & has no weird lumps, bumps or skin changes-i think you can sit tight and monitor the pain over time. Has there been any trauma or strain? Keep track of the trends in pain over the next 1-2 days. If it worsens or persists you should have him evaluated. (how old is he?).