First, his cost basis is the price you originally paid.
FMV is only important if he placed the vehicle in service for his business in 2021 using the actual expense method. In that case, the basis for depreciation was the FMV at the time it was placed in service, or the cost basis, whichever was lower. But, assuming he used the standard mileage method, he did not need to know the FMV.
When the vehicle was disposed of, he would owe capital gains tax if the selling price was more than the adjusted cost basis. The adjusted cost basis would have been your cost (in 2010) minus any depreciation he took either as a specific expense, or rolled into the standard mileage amount (generally, between 25 and 30 cents per mile of the standard mileage rate is a depreciation allowance.)
I believe Turbotax should include this calculation, it should ask for his basis and his depreciation or mileage. His basis is your purchase price from 2010, not the FMV on the date of the gift.
