My son accidently made a serious scratch in disk #1 on his licensed copy of GTA 5 and The game can't be installed on his pc anymore. Is it possible in any way to buy a replacement disk in order to complete the installation, and in case, how do I get one? Or do I need to buy a complete new version of GTA 5 for him?
While they mention in a couple of their Rockstar Support Articles about a disc replacement policy, I can't find anything specific about it. You will probably need to contact Rockstar directly and explain the situation.
However, you can use the Rockstar Activation Code that comes with the game box to redeem a digital copy of the game. Below is the link to the Support Article, along with the relevant Q&A.
Question: If I buy a new boxed version of GTAV PC with DVD discs, can I use the Rockstar Activation Code in the box on the GTAV PC Social Club Activation page to download the game instead of installing from the discs?
Answer: Yes. When you use a Rockstar Activation Code to activate a copy of GTAV PC, your Social Club account becomes entitled to access the game, whether you choose to install and use the DVD version, buy the game from a digital retailer that hosts its own GTAV download, or download from the GTAV PC Social Club Activation page. Also, the Rockstar Activation Code included in a new physical version of GTAV PC can be used to download the game from Social Club (for example, if your Internet and DVD speeds mean downloading would be faster than installing from discs; or, if you accidentally damage one of your discs in the future).