Fortunately for you, there are a ton of kid-friendly drones to choose from. In fact, that's maybe part of the problem: With so many available at a wide range price points and technical abilities, it can be hard to know what's appropriate (and worth your money).
Without knowing how old your son is or what in particular excites him (racing? taking pictures?) I would suggest looking at the following options, each of which has their advantages but are also good all-rounders.
Eachine makes a number of affordable mini-drones that are good for learning how to fly. They're robust enough to take a few knocks and come with protectors so the propellors won't nip your son's fingers (or get caught in your hair -- it happens). Bear in mind these won't hover in place; if you take your hands off the controller it won't stay in the air. Also, battery life is short: just five minutes or so. But, this will teach good piloting skills that will carry over to more advanced models if he decides to stick with it.
If you want something that's easier to get the hang of and don't mind spending a bit more, Parrot's Mambo wins points for its features, flight time and kid-friendliness. The Mambo has accessories like a camera or mini cannon, and can hold itself in the air (and even do tricks). Parrot's app also lets you see what the camera on the drone sees, and even comes with a pair of goggles so he can "feel" like he's flying.
If you're feeling flush, you might consider the Spark from DJI. It's pricey for a first drone, but flies like a dream has better battery life (about 15 minutes), shoots impressive video and is small enough for young hands (you'll definitely want the propeller guards on this one). At $399 it might seem like a lot of money, but as your son grows up the Spark won't feel like a toy so he'll still be able to enjoy it.