My research says that Form 2106 is not allowed in 2018. My son has been
deducting his mileage expense in previous years, but I keep running across
that this is not allowed in 2018. Where can he deduct this expense?
If he is a W2 employee then no, employee expenses are no longer deductible under the new tax laws (for tax years 2018 - 2025).
If he is a statutory employee or received a 1099 then they are deductible and reported on Schedule C along with his income.
[Edited 3/11/20|8:40pm]