For the employee, the company withholds income tax, Social Security, and Medicare from wages paid. Those are also called benefits.
Also even for 100% commission, W2 employees must still get minimum wage. If the commissions would be less, the company needs to kick in the difference (thanks to Kathy for this extra information).
In general, the FLSA does not restrict the forms of “remuneration”
that an employer may pay—which may include an hourly rate, salary,
commission, piece rate, a combination thereof, or any other method—as
long as the regular rate is equal to at least the applicable minimum
Another benefit, which is very relevant in this case, is that if he doesn't get paid one week for the wage he's owed, he can go to the Department of Labor of his State and they'll make sure he gets paid.
An independent contractor, on the other hand, doesn't have that option. He would have to go to Small Claims Court, which costs ~$60 and which could take many months.