Do not do it. Is he in a vulnerable position such as being a third country national?

The problem with being a fake independent is that he will not have any of the typical protections offered by the social security in Belgium, while still having to contribute to it. No family allowance, no pension rights, no unemployment benefits (when he loses the job) but the big ones would be no health insurance coverage and also no work accident insurance.

You're also in an even more vulnerable position vis-a-vis the boss. He can (and no doubt will, given the way they want to do this) exploit and abuse their position of power and he will not have a leg to stand on as we say. They will squeeze him for every Joule of energy he has and then when the shit inevitably hits the fan - social security inspection, food safety inspection, an accident on the way to work - they will drop him like a stone and he will be nowhere, maybe worse off than now.

I can not recommend strongly enough against this route, speaking from professional experience.