Possibility #1:  They are Self Employed.  Report on Schedule C.  This is how the business that is paying him is treating him.

Possibility #2:  He is a Statutory Employee.  A SS-8 should be filed, and he pays FICA on Form 8919.  Schedule C would report any expenses.  But "traveling sales" doesn't necessarily mean he is a Statutory Employee.  And DON'T let the client TELL you that he is.  If you think the 1099-MISC is wrong, then YOU need to make that determination.

Possibility #3:  He is a 'regular' employee.  A SS-8 should be filed, and he pays FICA on Form 8919.

https://www.irs.gov/businesses/small-businesses-self-employed/statutory-employees

Form 4137 is absolutely wrong.