My 45 Yr Old Husband Has Been Suffering From Lower

My 45 yr old husband has been suffering from lower back flank pain that he describes as burning around the kidney/flank area for about 5-6 days now accompanied by fever that seems to develop more acutely in the Evening time. 4 days ago he was diagnosed with steep and has been on amoxicillin since then. He was told it was a muscle strain, UA negative for UTI. They did a back X-ray and it was also negative. To alleviate the pain ease up he takes 8 ibuprofen. The last two days have been tolerable until evening time when the pain and fever both heighten. Any ideas what it could be?