It's a bit on the early side. Things change with them, so he could end up learning all of this content for the course content to change. I started learning the content year 9 for triple science, maths, and a couple of other subjects. The rest weren't even started until year 10. That being said, if he's eager to start learning ahead then that's great and should be encouraged. If he can handle the extra workload, then there shouldn't be an issue. He might be able to get his teachers to support him by marking the odd piece of work, and he can study and revise online. He needs to be determined though, because that's a lot of years of GCSE revision if he starts in year 8 and it'll get incredibly repetitive and annoying to do