Thanks for writing your query.
After reading your post, it seems that he is having spasm of the muscles of the lower back.
Is there any history of fall or injury?
He can take pain killer like Ibuprofen
one tablet twice a day after meals along with muscle relaxant Tizanidine
2 mg and antacid Omeprazole
20 mg to decrease the pain. Medicines should be taken under medical supervision only.
Doing hot fomentation and application locally of pain relieving gel will also be beneficial.
If the problem persists, he should get examined by an Orthopedician and get a MRI of lumbosacral region done.
I hope this is helpful to you.