My brother received a corrected w2 after he filed his taxes last year, but he didnt amended his tax return. how does he have to do this year?

He should still amend his return from 2017. He has up until 4/15/2021 to amend a 2017 tax return and claim a refund.

He cannot add this corrected W-2 to his 2018 taxes. 

Also, he can still file his 2018 taxes without having completed the amendment to his 2017 return.