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It is essential to understand the cause of a concentration problem. Based on the limited information provided here I can think of a few remedies.
If your son has taken too much stress of his studies and due to which he is not able to perform well or unable to concentrate, then you can give him Kali Phos 6x two tablets twice a day for 15 days. If Kali Phos 6x is not available, then get Phos 30c three pills once a day in the evening for seven days.
You can give him Five Phos 6x if he is generally not able to concentrate and the cause is not known. Five Phos 6x two tablets can be given twice a day for 15 days.
I would suggest you seek help from professional homeopath either face to face or online consultation on icliniq.com. It is very important to understand the details of his problem to prescribe any homeopathic remedy correctly. I hope he will feel better with this remedy.
Let me know if you have any questions or else report back in 15 days.