In addition to his role at the UU Willem currently is:

- co-author of the legal commentary 'Tekst & Commentaar Aanbestedingsrecht';
- editorial board member of Tijdschrift Aanbestedingsrecht;
- member of the curriculum committee of the Rijksinkoopopleiding (RIO);
- chair of the Advisory Committie Proportionality Guide (appointed from 1 September 2022);
- board member of the Stichting Utrecht Law Clinic;
- endowed professor of public procurement law (chair by the Dutch Association for Public Procurement Law).