You use Actual Expenses (fuel, insurance, depreciation, repairs, etc.) *OR* the Standard Mileage Rate.

However, are you saying he now has a total of six vehicles?  If five or more are used at the same time, he does not qualify to use the Standard Mileage Rate.  So if that is the case, just forget about the Standard Mileage Rate.

So *IF* he can't use the Standard Mileage Rate, your only 'choices' are how to depreciate the new vehicles.  Bonus, 179, 200%DB, straight-line, etc.

My personal opinion:  Unless bookkeeping is your main business and tax just a minor aspect of it, you should not be adding up all of those receipts this time of year.  I would tell the client I would only do that in December, January, or file an extension and do it in May.  Or have him sort through them and add them up.  At any rate, be sure you are being paid well to add all of that up.