What should I do to improve my son's concentration in studies?

Hi doctor,

My 14 year old son cannot concentrate on studies and struggling in school. He is nervous and shaky, especially in school. He scored 65% overall and 85 percent in his favorite subjects. He reads a lot of story books. We went to a psychiatrist last year and he said that my son is suffering from anxiety and gave Sertraline 50 mg initially, after 15 days increased the dose to 100 mg and after six months, he increased it to 150 mg daily. He was taking 150 mg of Sertraline for the last eight months. His anxiety gone, but there is no improvement in concentration and studies. He feels tired after 10 to 30 minutes of study. So, he struggles a lot in school. He is a quiet and shy boy. Please help him. There is no assessment done yet as he refuses to go for that. He says that he does not need that. He writes very good English and solves many computer science related issues, but his score is very poor in other subjects. I do not understand what is wrong with him. Is it good to give the medicine to one and a half years? Is that affecting his intelligence? In lower grades, he used to score 80% to 90%. So, he himself worried about his poor marks. He wants to study well, but he is not confident to manage his curriculum. What should I do? We always encourage him and do not stress him on marks. His birth was normal and there is no family history as such. Though he afraid of his father as he gets angry, he has a good relation with him. But he clings to me more. He has very good bonding with his elder sister. He also had a counseling session for nervousness. He had an MRI of brain, blood sugar and EEG. Everything was normal.