It depends on the form he receives from his employer.
If he gets a 1099-MISC, then he is considered self-employed and will file a Schedule C. On that, he reports the 1099-MISC, plus any other income he's received and takes expenses, like mileage.
If he gets a W-2, then he can take his business mileage only as a miscellaneous deduction subject to 2% on your Schedule A.
For more on this subject (because it can be pretty complex), see here.
