I found getting rental accomodation in the Netherlands quite tough, and imagine a town like that may suffer from the same difficulties as the one I’m in (i.e. not Amsterdam, Rotterdam, The Hague or Utrecht). There weren’t many properties available to rent full stop, yet loads being bought and sold, I know that people are far more credit averse here.
I found the expat groups on Facebook and Pararius were my main sources for this sort of thing, but don’t know if your town has something specific about it that makes it easier. There’s Marktplaats & Funda which are worth trying too.
Sorry I can’t be more helpful, looks like you’re doing most of the necessary things, but best of luck to your son!