As far as building experience, LEGO Technic sets are considered the most challenging. Looking at your list, it appears that your son already built some pretty large LEGO Technic sets, so if he enjoys the challenge, you can purchase him more of the larger Technic sets. Also, note that most large LEGO Technic sets will have alternate instructions, or the models can be combined with other sets. Additional instructions are available on the LEGO Technic website.
Compared to LEGO Technic, pretty much all other sets are going to be simpler, however some of the larger regular sets can still provide a good challenge. I would suggest browsing the paper catalog or LEGO's online shop together, and see what strikes his fancy. He might be into a particular theme to not just build but also play out stories. Or he may like vehicles, trains, motorized sets, or robotics.
Also, encourage him to free-build. There are alternate instructions available for many LEGO sets both online and in book form, or he can just build from his own imagination. That's really the next level for a talented LEGO fan; designing and building their own models.
You might also want to check if there is a LEGO club near you for young LEGO fans. They are usually well structured and focus on robotics and other educational aspects of LEGO, which may be a good fit for your child.