At that age we mostly stuck to The Magic Faraway Tree and other of the magical Blyton books - he was reading independently but, yes, needed something not too scary or too far outside his understanding. There are OODLES of Blyton books that are about right for that age / level. He also really started enjoying Encyclopedia Brown about that age, he also fell in love with Charlotte's Web, but of course that is VERY sad. Some of the (original) Thomas the Tank Engine stories might work too.