Unless he is being Paid to be a rolling advertisement for some company, then the Use of the vehicle has nothing to do with business, just because there is a sign on it. The use of the Vehicle stands separately from whatever sign is on it, unless Advertising is the income stream.

I agree 100%, if you google that subject you will see court cases of people that tried that, and was unsuccessful

That's funny, I saw one of those magnetic signs the other day and it made me think of the old days when the unsophisticated self-employed thought it was a magic way to convert personal expenses to business.  

One of the long running tax deduction myths!

Publication 463:

Advertising display on car.

Putting display material that advertises your business on your car doesn’t change the use of your car from personal use to business use. If you use this car for commuting or other personal uses, you still can’t deduct your expenses for those uses.

https://www.irs.gov/publications/p463#en_US_2019_publink100033920