A few of my kids at school read fantasy novels, but none were hard core science fiction fans. In our reading group, I introduced them to Robert Heinleins juvenile novels. They were hooked!

First we read Tunnel in the Sky. They wanted to read more. Since the middle schoolers were studying the universe, we went on to read Time for the Stars. We finished with Citizen of the Galaxy. I had a hard time getting them to read aloud, since about half of them wanted to read ahead. They truly enjoyed the books, and learned a great deal about science and the societal mores.

I've heard that Heinlein is great for that age. I'm still trying to get my 11 yr old to read him, but you know how that goes-- If mom thinks it's good, it can't be

He's an advanced reader so it's always hard to find something challenging that he would enjoy, but that's not too mature for him. Although, I'm not sure if he'd like Heinlein. He loves non fiction science, but his fiction tastes seem to be on the silly side (he really enjoys Terry Pratchett)

Sit down and read the first book together. I'm sure that the kids in my class would have given Heinlein a pass, but they read him with me. Since your son is an advanced reader, he should appreciate Heinlein. The vocabulary is challenging and so are the scientific concepts. Heinlein doesn't talk down to kids.

I recommend Infinite Possibilities. There are three books in it. All are popular with young men. You can get the hardcover in a used book for under $8 including shipping. Get one for each of you!

Here is a link:
Infinite Possibilities used hardcover