And not only that, but a verbal contract cannot modify a written contract. At least in US law. So he is only bound by what is written in the contract he signed, not what was said outside the bounds of that contract.
In the German system the oral addition stands above the written contract. The written contract is considered boiler-plate by the courts and the oral addition is more specific, and bears more weight.
Of course all parties have to agree to amend a contract. So just the employer changing their mind wouldn't cut it.
The Netherlands is a civil law country, so the German standard might be closer to what they have than the common law American standard.
Labour relations and law are of course a strange thing in themselves.