Distance learning - restrict certain programs / services?

My challenge is as follows.

Our youngest was given a PC in his child's room due to the Corona school closure and of course with an internet connection.

Unfortunately, he uses the computer not only for teams / wiki etc. But also to my chagrin for the favorite game of the whole class (Fortnite). Now I'm of the opinion that in addition to the whole school environment there must also be a certain amount of leisure time, so I have nothing against occasional games.

Question:

How can I limit the PC for learning tasks indefinitely, but for each type of game? Please no references to… Just look regularly into his room or something similar…

I use a Fritzbox 3490.

There's no easy solution.