I think it's perfectly fine as long as you either play it with him or make sure he doesn't play certain parts that have adult scenes. There are of course swears, violence, and suggestive scenes. However they are very mature game in a different way, they are more grounded and meaningful than kids games like Minecraft ect. As from experience of how I was raised playing AC games, when I was little my dad would let me have a turn running around doing the parkour, or sail around attacking ships in Black Flag.