Just played through actually, thoroughly enjoyed it. Husband and I are both in agreement that our children can play once they leave home.
Rockstar the games developer's niche is that they create movie style games usually where you play the bad guy. Regardless of how ethical and upstanding you try to be missions will force you to torture a man (graphicly), murder innocent people, and commit some pretty horrific acts that even we found somewhat scarring. The problem with the game is not the prostitutes (or the strip clubs you are forced to visit) but that the entire storyline is geared towards behaving like a criminally insane psychopath, in a highly immersive world where your horrific actions feel justified. The game is satire, and enjoyable a such, intended as an exageratted extreme view of american action movies which glorify violence and often feature criminal protagonists.
The same can be said, to a less extreme extent, of rockstar's red dead redemption and (showing my age) bully franchises.
Other game studios tend to create a more balanced game, you can be good or evil, missions involve options which allow you to choose how you behave and who you hurt. All games tend to feature deep moral questions, which can be difficult even for an adult, but most will allow you to be a hero rather than forcing you to be a villain.
As an alternative consider fallout 4, Skyrim, the witcher, Dragon age, or even red dead redemption. All great open world, storyline based, very popular games like gta only in these you are given more free choice about how you play. Games are aimed at adults, all of these titles include adult themes, prostitution, murder, and difficult moral decisions, for instance the ending of fallout 4 where you must choose one of four factions to survive and have to kill the rest (after you spent the game helping them and being welcomed in to their groups).
Whatever game you choose I recommend a console over pc, solely because it makes the game more open being on a television rather than hidden away on the computer, and actually watch some of the gameplay. Keep communication open, even if it's just in a lighthearted manner where he can joke with you about some of the issues that come up in these games.
Disclaimer, I do love rockstars games and have played all of the games I recommended, but have not yet finished the witcher or Skyrim.