I have a feeling the OP will not be mentioned on the companies books as an employee, meaning there is no trace of him which means its not illegal to not pay him.
If he has signed a contract of some kind then yes it will be illegal not to pay him a minimal wage.
I would be careful on that!! I would personally tell the company I would work for them at least 2 days a week and the rest I will work in another profession because they are not paying me, and if they want me to work more than that then they will have to start talking money.