281,000 miles would have much more than $5000 of fuel cost.

And are there more than the three vehicles that he bought this year?  That would be a lot of miles for just three vehicles.  And I hope he told you the miles for each vehicle separately.

Did the client hire you to do his bookkeeping to add up receipts?  I hope you are charging him well for it.  Generally, for just the tax return, you ask the client the amounts and they need to give you the total amounts.

What types of vehicles and what are they used for?  Do they qualify for the Standard Mileage Rate, and if so, would that be beneficial?