Best thing is to trawl the internet and try bits of everything. CEM is Verbal Reasoning (VR), Non-Verbal Reasoning (NVR) and Numerical Reasoning (NR). In short, it is something you can teach your DC, yourself.
I am presently trawling the internet for materials, and using them on a friend's Year 5 DS. Friend's DS will sit the Warks 11+ in November 2010.
VR is heavily dependent on a wide vocabulary - both in knowing the words, and what theyactually mean - especially homonyms. It also will include a cloze exercise - there are many on the internet to look at. Alternatively, copy paste a paragraph from say Times Online, or First News, and remove key words, such as prepositions. Ask your DC to complete them. There will be questions on synonyms and antonyms, again, vocab is important. Comprehension will be multi-choice, so Try the Eng Bond Books and the Comprehension Bond Books. ABOVE ALL, DEVELOP THE VOCAB!!!
NVR, is exactly what it is. Use the Bond Books, and the two CD's on the Eleven-plus forum.
NR - This covers arithmetic and problem-solving. For these, best to get your son to used the KS2 publications in Arithmetic and Problem-Solving, by Schofield & Sims. They are ideal preparation.