Forgot to claim input tax credit

Dear Friends

Mr. X filed nil return GSTR 3B for May 2018. 

He had one purchase in month of May 2018 but he wrongly filed Nil return GSTR 3B.

Instead of filing nil return GSTR 3B he should have claimed ITC on this one purchase he had.

Now how can he get this ITC which he forgot to claim?

regards

Shivani