"i realize a mistake was made on his 2019 return"

Just how much Business Use did he report specific to 2019, for one date?

https://www.irs.gov/taxtopics/tc510

"To use the standard mileage rate for a car you own, you must choose to use it in the first year the car is available for use in your business. Then, in later years, you can choose to use the standard mileage rate or actual expenses."

https://turbotax.intuit.com/tax-tips/self-employment-taxes/standard-mileage-vs-actual-expenses-getti...