One I was little I was fascinated by comics and my mom would regularly bought me some that came in long histories format. The incentive was such that I became fast at reading, and I realized of that in high-school, when I had to start reading more and I found I could do so well and fast. Even then I realized that it was because of my early devotion to comics.

I repeated the experience successfully with my little brother, who was having a very difficult time at studying. So I started buying him some long-story format comics of a style I knew he would like, and he did. Later he himself remarked to me that that helped him to get started at reading more easily when studying and get up to rhythm.

So my advice to you is to get comics that come in "book" format, so that they are long enough, and of a subject that also comes in books, so that your little will start with the comics, get used to read more and for longer, and then he will have the incentive to transition into the books himself out of wanting more.

I'm taking yours as well for the future, it's fascinating we can teach kids from a young age with this tools, the next generation will be super-human.