Why Doesn't My Son Listen To The Teacher And Not Write Sentences Correctly?

Hi, My son's age is 9+ he is the only child of us. He doesnot behave properly in the class. He lays on the benches, rocks his chair doesn't listen to his teacher. He doesnot feel writing in the class. he sits with his pencil and copy. he cannot play properly with his friends. also sometimes he cannot write sentences correctly. at a younger age he has been diagonised with delayed milestone. he reads in class 3. he can sing well. he loves to hear bengali songs. he is very free in home. he is very well behaved when we go to shops. he does self talking. he repeats many sentences or words.