VAT Returns

Just google to find My MinFin.
With his Resident Permit card and a E-Id reader, he will be able to fill it from within MyMinFin.

By the way, the latest delay is the 20th of January....

Yesterday the website was already down (too much connections).

https://ccff02.minfin.fgov.be/portal/fr/public/citizen/welcome

But are you sure he will do that without being helped by his accountant? Fiscal laws are extremely complex in Belgium and fines are huge...