I'm sure it's all fine, and either a fluke of that test on that day for some reason or other, or something that will be easily put right with a bit of extra reading. I've rambled on a bit below with some other potential solutions, but my gut feeling is you will be posting in two weeks time saying you find some great books he loved reading during the Easter Hols, and you're pretty sure his reading age is now about 15.
I'm probably being unfair about the school, but if your school is like mine, I think that if you did not read much at home, and once you were past the initial beginning to read phase, you could go to school all day every day and not boost your reading age. Reading in any shape or form does not seem to figure much at DD's school, and if you had been placed in a lower group for whatever reason, the written material that would be put in front of you would be of a fairly low level.
It sounds from what you say that he was an early reader, so hopefully it isn't some kind of "de-coding" problem. But if it was and you wanted to give it some kind of quick fix then you could try a book called Toe by Toe and carefully pick some exercises from it yourself. It's not intended to be used that way, (one is supposed to work through it from beginning to end) but if for example you think your DS2 is guessing wildly on more complex multi-syllabic words some of the nonsense word exercises in there will help.
It is a very thorough book that is designed to get all ages learning to read from scratch up to a very high level. It is what is used in some schools when children hit some kind of reading problem. But it is not Reading Recovery. Toe by Toe is phonics based, not multicueing. There's a lot of controversy about whether Reading Recovery (and I don't know if your school uses the word Reading Recovery loosely or not, but the true Reading Recovery is not phonics based) is a good programme or not. If your son hates Toe by Toe and the reading age was just a fluke, I would use it very sparingly. But if he quite likes it, Toe by Toe can boost reading ages very significantly in a short space of time, without your child ever having to read a book!!
You can read loads of reviews of it on A*****.
The other method of getting a quick boost in reading age is as you say by reading. If you read posts in TES primary forum by a former headteacher and educational researcher called Eddie Carron you will see that he has developed a free CD which has on it a lot of interesting reading matter. He calls it an Electronic Library. He has done this as part of a research project for schools as he is convinced that if schools use his method 97.5% of children could be level 5 readers by the end of KS2. He might be able to send you the disc, or someone else on there who has downloaded the disc might be able to send you their old one. He recognises that some children are motivated by the reading being computer based. Once children are enjoying reading on the computer and have improved their reading ages he reckons they will be far more motivated to use a real library.
Another thing I have seen stunning results with is a boy who got very interested in reading and massively increased his reading age in year 5/6 through following books which were also on CD. He did it with ?Artemis Fowl? . In the first book of the series he could barely read them. But then by following on CD he just massively improved. He then got to a point in the series where it was not available on CD, and as he was desperate to read the next books he did so without the CD. It can be difficult buying books on CD as sometimes they have been abridged which is useless for following from the unabridged books. It gets expensive with much longer books too. But hopefully it's a stepping stone and one you need only use for a brief period.
Does the school do annual reading age tests, or do they just do a few random things when the fancy takes them? I got a print-out of part of my DD1's school assessments at the last parents' evening. Reading age was on there at the beginning of year 2, but not in year R or year 1. So I don't know if it was a random one-off, or part of a new and regular system, or just something they do from time to time for the children of nightmare parents