Obtaining a copy of the Will of a Belgian citizen who died in 2020

Easiest thing to do is contact the notaire who is handling the estate.

Remember that he may not have made a will. Belgian law provides a fall-back situation that suits plenty of Belgians so they don't go to the bother (and relatively minor expense) of making a will.

There are strict rules under Belgian law detailing what you can do with your estate so many (most, even) don't make a will but, whether or not he has made one, there will still have had to be a notaris involved.