A "client" of a self employed sub contractor cannot demand you do anything ... you and the client agree on a contract for a fee.
So if you do not like the terms of the contract you have then fire this client and get others. So if you don't like the situation find another OR turn the "employer" into the IRS by filing the form SS8 if you believe you should be treated as an employee and not a contractor :
https://www.irs.gov/taxtopics/tc762
