Contractor Among Contractors
I can see two approaches:
1. A written notice that fees are increasing.
2. A written notice that an employment tribunal is going to look askance at them not giving him employment rights / holidays if they are going to declare that he is an office holder or has the same working practices and does the same work as an employee.
The former might get his contract terminated, depending on how much he asks for. The latter almost certainly will get it terminated.
Other approaches might be acquiring compromising pictures of the person using the HMRC tool, etc.
If they are calling him an office holder and he's not, then they need to get someone who actually knows what is going on to use the tool.
If he has multiple clients and most are fine, he could do the inside contracts inside an umbrella and pay his salary and pension contributions through that.