@Joseph **********
nope, you can't switch the type of Non-Immigrant visas inside the country.
You can switch reasons for an extension BUT he's NOT on an extension he's on the original 90 day entry of the visa itself.
He can't use this visa to get an extension based on employment.
he's gonna hafta take documentation from the company he's gonna work for and go get a 90 day single entry Non-B visa to start the process of getting an extension based on employment