Box 18 on a 1099-MISC reports the state income. I am assuming you previously received a 1099-MISC from him. If you already reported the state income on your state return, there is no need to file an amendment, and no, it should not be a red flag to the IRS since it is not reporting anything different for the Federal return.
Just check and make sure the income flowed through to the state, as it should have if you reported the income (originally) on your Schedule C.
Keep a copy of this 1099-MISC with your tax file, but if the only box reported is box 18, and that income was already reported on your state return (even if through your Schedule C) there is nothing more you need to do.
