"The company he works for reimburses him monthly for leasing the vehicle and mileage."

Tell him you can't double dip. He already gets the benefit from his company of being paid for what he wants to then write off. Explain you don't write off a truck as if that is printer paper. Explain mixed use vehicles.

"He states he has only been reimbursed $3375, and he has about $10872 in expenses. I believe it states only Army reservists can use the form."

Did you read Why they would use it: "and include the part of the line 10 amount attributable to the expenses for travel more than 100 miles away from home in connection with your performance of services as a member of the reserves on Schedule 1 (Form 1040), line 12, and attach Form 2106 to your return."

He's an employee of a business that already reimburses him for the business use. He might also be in the reserves, of course, so that related expense for reserves is why you use Form 2106 as a reservist.

There is no Free for All for using the same expense over and over, just because you find different provisions.