My son didn't have one of his W-2's by April, so I filed an extension for him. He didn't make enough $ to have to file. He didn't get W-2 so he didn't file. Is that ok?

How much did he make and why didn't he get a W-2? Employers are required to send out a W-2 regardless of how little the employee made. Lack of a W-2 indicates that the "employer" may have been treating him as a contract employee. Such "employers" are required to issue a form 1099-Misc (the amount in box 7) if the "employee" is paid $600 or more.

While dependents with W-2 income do not have to file if their total income is less than $6300, contract employees are considered to be self employed and have to file a tax return with as little as $400 of income, so that they can pay their social security and medicare tax, even of they did not get a tax form (W-2 or 1099-Misc) reporting the income.