I was about that age when I first read the Fowl books, and they were some of my favorites! There were a few concepts that I didn’t fully understand at the time (e.g., I wasn’t sure what the Russian mafia was when they discussed it in book 2), but overall there’s nothing horribly traumatizing for a child in there. Your kid will be fine!
There's still the occasional image, like in the series, the fairy language is read in a spiral, so there's an image of it with a simple cipher so they can translate it themselves, and other bits like that.
The characters are funny, yet the plot remains serious.
While someone did point out there's some serious things in there, they're not actually explicit, it's not worse than any other book. Harry Potter has murder, slavery, violence, Ave nazi-like cults, but that doesn't mean it's an adult-only book. It's about the presentation of these themes. Artemis Fowl IS fairy Die Hard, but it's not rated R, it's written for kids, and I highly recommend it for yours, it was perfect for me at that age.
I haven't yet either but I've read his Cytonic stuff and while it's not young enough, it's younger. I imagine the Alcatraz one is really well done. My buddies 8 yr old read them and loved them.
If they like Artemis Fowl, I suggest a series by Suzanne Collins next. Gregor the Overlander is the series she wrote before the Hunger Games, and I remember enjoying it when I was around 12 years old
Been a qhile since I've read it, so I'm not fully certain. I believe it's fine, but if you're uncertain maybe wait a year. The similarities to Harry Potter aren't too big, so I don't think that is much of a factor. Then again, it has much more action than Diary of a Wimpy Kid.
Keep in mind as well that age appropriateness can change from kid to kid. It kinda depends on what things he's been exposed to and been fine with in other media (i.e. Movies, games, etc.). Our input should help you make a decision, but we can't make the decision for you.