Forgot to claim relief u/s 89

I have a query regarding the claiming of relief u/s 89(1). The tax payer is a government employee who has received arrears of salary in FY2015-16 for the past years.While computing the tax liability for FY 2015-16, no relief was considered and the income tax for the FY2015-16 was filed by March 2017. There was no mention of the amount of relief u/s 89 in the tax return (Please note that the return not filed before due date but before end of assessment year). The return was a refund return and the tax payer received the refund amount from the IT department. Now the tax payer realised that he has not claimed the relief u/s 89 and not filed the 10E. Can he file the 10E online now.? Can the tax payer file a revised return since his assessment has already been completed and refund received? Had the 10E being filed with original filing, the refund amount would have been higher. Can anyone in the group suggest a solution. Normally very few people have good knowledge about the 10E filing. Sandesh