It may be an EU passport but since he is a German citizen, they still decide who gets to keep it and who doesn't. Even though they have introduced this clause about being able to apply to keep it, you must show that it would be hardship for you to lose it. Some other countries in the EU allow dual like Ireland and the UK and some others but it is up to each country.
For somebody who has lived in Canada, maybe for years, and only comes to Germany to visit, they could still come visit with a Canadian passport since they are visa exempt so it would be hard to show that it would be hardship for them to lose it.