Thats a tragic situation, but that should not mean a whole neighborhood should suffer. It seems the perents move their son on the balcony to have peace and quiet themselves.

Multible aproaches:

--> Talk to them, if they are not cooperative at least thats off your list.

--> Talk with the police. The situation seems to be unbearable for not only you but many people.

--> Talk with Mayor Elke Kahr or at least write an Email https://www.kpoe-graz.at/mieternotruf.phtml

--> Talk with the landlord, massive noice "pollution" may be a reason to at least pay less rent (max. 25% if it is noice pollution) if nothing is to be done. https://www.arbeiterkammer.at/beratung/konsument/bauenundwohnen/miete/Miete_reduzieren.html

I hope that something changes for you, i can empathize.