How old is your son ? While RDD is not as nihilistic and amoral as GTA, it's not that far behind.
Anyway.
The best thing about RDD is the open world and the stuff you can do in it outside of the main story quests. You have the "stranger" sub-quests where you find various strangers around and get involved with their stories, the bounty hunting missions, the specific area challenges where for instance you get into shootouts in abandoned towns with desperados, the animal hunting challenges that eventually lead you to hunting the legendary beasts, the various events that randomly occur along the way where you can help strangers in need, or hunt convicts, or get ambushed by bandits. Finding and taming horses was also pretty fun, trying to discover that really good steed among the rest. Herding indeed were the only boring missions, but at least they were few.
My all time favorite was the treasure hunts where you searched for pieces of treasure, by using clues from maps. The cool thing was that you needed to find certain landmarks and decipher where exactly the treasure was hidden. The exploration and clue-deciphering felt great. And that brings me to best feature of RDD, which is the world structure. It just feels great to navigate the setting. Each area has its landmarks, its wildlife and flaura, and it really feels like the wild west. You can fast-travel between regions, but most of the time I preferred to travel the landscape by horse because of the great atmosphere.
And the goddamned Texas hold'em was fucking great. I was amazed at how good the AI was for a what is essentially a minigame. I spent a lot of time playing poker in RDD and fact that you can also cheat, and get caught, was pretty fun. The rest of the minigames were subpar.
The writing is top-notch, especially the dialog, and it serves as to put you into the general atmosphere of the time and place, which is one of the last untamed regions of America where the government is trying to move in and the bordering Mexico, which is in constant revolution turmoil. I had some problems with the main plot at times, but all and all it was pretty good. Most of the characters were interesting and well written.
The gunplay was solid, as much sold as it can be in a console game.
BTW: When I'm talking about exclusives I basically mean console exclusives (= not PC)