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Thank you for your question, this can be sciatica pain, muscular pain or renal pain...
If this pain increases with raising his leg straight up while lying on back then it is most likely sciatica.
If he has burning urination then it can be a kidney infection or stone,
If this pain improves with pressing the pain area then it can be a muscular pain.
You can try medicine like tylenol and advil for pain, If these do not help then you have to take him to emergency. Specially if he gives symptoms of loss of urinary or bladder control, then this is an emergency.
Let me know if you have questions, thank you.