While you might want to query your son's progress with his teacher, you shouldn't necessarily assume that he's not being sufficiently challenged with his reading. Published schemes for teaching any subject (whether it be reading, mathematics or anything else) only provide a framework for the teacher to work with. Sometimes children require 'confidence boosting' materials. Some children will get a boost to their confidence by being allowed, for a short time, to work with materials which are slighly below their normal level of attainment. (By doing this, the child meets with repeated success and can overcome any short-term worries). Other children will get a boost to their confidence by being given the challenge of trying out materials which are above their normal attainment levels. A good teacher never works 'slavishly' to a published scheme. The best results can only be obtained if the teacher takes into account every child's individual needs.
The chart showing how the Oxford Reading Tree is organised is here:
http://www.oup.co.uk/pdf/oxed/primary/ORTParen tGde_0603.pdf
Note that that tree does not show the series titles (such as 'Owls' or 'Magpies') for the fiction materials on the 'trunk'. To see those, together with the associated reading ages (and the suggested school terms for each stage), go here:
http://www.oup.co.uk/pdf/oxed/primary/ORTreadi ngAges.pdf
Chris