I have 3 dc who were good readers, all got 4a s at the end of year 3 and progressed rapidly upwards.
Dd1 read ahead and once she'd moved on to the next level never went back. Her favourite book in year 4 was Watership Down.
Dd2 read a mixture. She's 15yo and I may still find her reading Rainbow fairies, which she first read over 10 years ago. In year 3 she read every level from Rainbow Fairies through to books like Anne of Green Gables series and What Katie did etc.
Ds liked to read factual books to himself. He was not interested in fiction if he had a choice, but when he was looking for a book in his interests he would go through the adult books because the children's ones were boring. However we read together various fiction books of all levels. As long as it interested him he'd keep up.
Now dd1 reads the least. Dd2 still likes to read. If you get no answer from her, she's probably stuck in a book. Ds still likes his evening story, but doesn't often read at other times.
I think actually rereading familiar books with simpler language is great. It teachers them to love reading. That the pleasure of curling up with a familiar book is greater than anything else. I love to curl up with a book I enjoyed when I was 8yo come to that.
What I found with dd2 and ds is reading the first bit to them works/ed quite well. With dd2 she got easily scared, so we'd often have "that's too scary". I'd read the first chapter, and usually by the end of the second page I'd be holding tight so she didn't wrest it off me to go and read herself. With ds, a sign he's really enjoying it, is finding that he's read ahead-or even finished it.
Don't try the Cherub books yet. He may be able to read them, but they're not for 8yos unless you want them to read about drugs, smoking, drinking and sex (for example one of the books includes trafficking of a young girl for sex) . Mine read them around about 10-12yo with supervision (and I did read the books first, and some were definitely left until later!), so I could discuss and answer questions.
If he's interested in that sort of story, the Alex Rider are far more suitable for that age, and the worse you get in those are "Alex swore" 
Take him to the library and let him choose some, and perhaps suggest one. Ask him to see what he thinks. If he doesn't like it, then don't force him. If the library's got too many other things, then take time to let him do both.
I've never stopped them reading "younger books" but occasionally I have removed an older one-usually for a specific reason.