Apply Hot Fomentation twice daily. Avoid bending in front. Postural Correction- Sit Tall, Walk Tall. Extension Exercises x 15 times x twice daily - lying on tummy, take left arm up for 3 seconds, then bring it down, right arm up for 3 seconds, bring down. Bring right leg up, hold for 3 seconds, bring it down. Then right leg up and hold for 3 seconds and bring it down. Repeat twice a day- 10 times. Bhujang Asana -- Lie flat on your stomach, keeping the palms out, bend the neck backward, take a deep breath and while holding it for 6 seconds, raise the chest up. Release breath and relax your body. Repeat the exercise 15 times twice daily.

I assume you already had consulted, lot of doctors see if he has been tested for any disease which involves immune System. Tests like HLA B 27 or RA factor, If these tests are positive or even if these tests are not done Consult a Rheumatologist.

Consult the orthopedic doctor. X-ray and vit d  test . Start physiotherapy treatment and avoid long sitting. Posture corrections must.

He needs further evaluation consult orthopaedic doctor. Back pain with out leg pain usually settles with physio. Once pain settles do back muscle strengthening exercises. Drink plenty of water to avoid dehydration because dehydrated disc is more prone to injury

Exercise to Get Rid of Your Low Back Pain! - by Dr.Pooja Pandey Tripathi(PT) https://www.practo.com/healthfeed/exercise-to-get-rid-of-your-low-back-pain-32455/post

He needs to see Rheumatologist and also done vitamin D3 and B 12 done.Rheumatologist will be able to rule out any auto immune diseases.if he is normal by his opinion , only then physiotherapy should be attempted. If he has RA or AS or any of the Spondyloarthropathies physiotherapy will have to be dealt with appropriate medications and only appropriate timings.

Meet physio

He needs to see an orthopedic doctor and get further evaluation. After which he can visit a physiotherapist.