From what we see of the other Magnus' room, we can see he's into punk (flyers, texts on the wall). He also has quite a number of tattoos. He also has a prominent black X on the back of his left hand.
A black X on the back of the hand is well-known symbol of straight edge, a punk subculture that renounces drug use.
In episode S3E3, we see Ullrich ask Martha if they took any drugs when they found Mads, when Magnus comes down the stairs and is not amused.
[Ullrich] Habt ihr was genommen?
[Ullrich] Hat dieser Kilian euch was gegeben? Komm, was habt ihr genommen?
[Magnus] Was soll denn der Scheiß?
Did the creators of the series mean Magnus to be straight edge?
Does this add to Magnus' anger at his father's question, that his father didn't even realise he would not take drugs as a matter of principle?
Or did the creators just pick some symbol they knew to be related to punk?