I'm doing my son's taxes and his w2 is incorrect. According to his second week of October paycheck for 2017, his fica, ssi tx withhld, etc not correct. How do I proceed?

If the W-2 is incorrect contact the employer to get a corrected copy. 

The best course of action is to request a corrected W-2 from your sons Employer as soon as possible. If it's listed incorrectly it could lead to an examination of your return by the IRS. 

"If your attempts to have the Form W-2, Wage and Tax Statement, corrected by your employer are unsuccessful, you can request that an IRS representative initiate a Form W-2 complaint."

Here's a link to the IRS website with more information about W-2 replacement: https://www.irs.gov/faqs/irs-procedures/w-2-additional-incorrect-lost-non-receipt-omitted/w-2-additi...