My brother filled his taxes before he got a w-2 he wasn’t expecting.. is there a way he can add that w-2? or is it too late for him to file it?

If your federal tax return has been filed and accepted by the IRS, then you can only amend the tax return to make any additions or changes.

An amended federal tax return, Form 1040X, can only be printed and mailed, it cannot be e-filed.  The IRS will take up to 4 months or longer to process the amended tax return.  If taxes are owed after completing the amended return, you send a check with the amended return when it is mailed.  If the amended return has a tax refund, then the IRS will send you a refund check in the mail for the refund, the refund cannot be direct deposited.

The tax return should not be amended until the tax refund has been received from the IRS.

See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

He should file an amended return. Shouldn't be much of an issue as long as filed before deadline.