Deduction u/s. 89

Respected Sir / Madam,                                     One of my client will get Rs. 2.5 Lakh (appox.) as arrear salary from a.y. 2011-12 to 2019-20. Though his gross income was above taxable limit ( nil tax ) he has not filed his return for a.y. 11-12 to a.y. 16 - 17.    My questions -  1)  Can he claim deduction u/s 89 for the above periods ?           2) Is there any possibility by I.T. Department to charge him for non-filing of I.T. return for the above period ?                                   Thanks in advance.