It’s not unreasonable to prevent a 13 year old from playing an adult orientated 18 game.
I used to be asked if games were suitable for friends children, which combined my opinion plus parents guides to the games.
GTA was out of the question, but one of their friends was allowed based on him telling his parents that he ‘didn’t play the missions, only in open mode’
In my opinion that made the game worse - the first mission was to collect your bosses girlfriend, a prostitute and drive her to hospital in time for her STI checkup. Not ideal, but prostitutes were employees of the business etc
In open mode the prostitutes are someone to pick up, they restore your health and are an opportunity to kill and rob for money.
The games missions give context in your journey rising in a criminal organisation, but in open mode it’s about driving badly, evading the police, running over and shooting random people.
If you look up games for the parental guidance information and watch some YouTube videos on gameplay then you can make an informed decision on whether you are happy with the context and how mature you feel your son will be in (truely) understanding games /films and real life