Try it yourself before you buy it, and/or watch an IGN or related video game review. GTA V has been out for a few years now so there will be plenty of opportunities to rent or buy it used for a reasonably low price. Parents should ALWAYS play or thoroughly review Mature-level games before letting their kids claim ownership of one. If you feel he can handle it but you question the very immoral aspects of the game you've yet to encounter, play it with him. I'd personally recommend that you always play with your severely underage children, if/when they play mature games.
You're (sincerely hopefully) not buying this game to help babysit him while you're off doing other things, so, within your control, use every adult situation he experiences as a time to bond and educate. Video games and films are possibly the best scenarios to put this in context: you're in a private setting, away from judgmental adults, and in prime control of the situation. The toughest part about being a parent is dealing with the fact that your children will face (or have faced) adult situations without being prepared. I write this from personal experience; my mother allowed me to watch very graphic, R-rated (18+) films when I was a kid (as young as 10 [games hadn't become nearly as explicit back then]) but she did so only if I could tell her what to expect (i.e. reference newspaper reviews I had researched) and she had the time to watch it with me. She often grimaced at sex and violence but it always followed with an open and honest discussion (usually her discussing the bad parts and me discussing the 'cool' parts). Eventually, as I got older but still a legal minor, she began to trust my behavior and gave me more freedom to watch independently, but I distinctly remember her being there for many awkward movie moments.
Many parents will disagree with me, but as a parent of young children, a video game enthusiast (and, yes, I've played quite a few hours of GTA V) and a 'diagnosee' of ADHD, your child needs you more than they need alone time in front of a screen. I'm sure you're being honest with yourself: you know he's going to play it eventually (if he hasn't already), either with his friends or sneaking it in from who-knows-where when you're not around. Impress upon him your openness and awareness. Control the situation by limiting the time you experience it together. Discuss how you feel after or during each session and encourage open communication.
If you feel he's not ready for this game, I suggest a crash-and-burn style game like Burnout Paradise. It has lots of driving exploration, high-speed pursuits and big explosions without the sex, drugs and mutilations. There are more recent games with a similar motif but I personally enjoy it the best and go back to play it often with my kids.