My son received a W2 from his work with taxes deducted, but also 1099 without deductions. Does he have to pay Social Security tax on the 1099 and where do I enter the SS tax?

1099-misc income:
You are self employed or independent contractor. You must file your tax return if yourself employed income is $400 or more. You will report your income and expenses on schedule C or C-EZ (Form 1040). Then you put net income (or loss) on line 12 of Form 1040. This income is subject to SE tax at 15.3% (this is shown on line 57 of Form 1040). Half of SE tax is deductible on line 27 of Form 1040. Read: Your U.S. Tax Return: Tax Filing by Self Employed Sole Proprietor or Independent Contractor