My DS is about to sit for Reading School so I've been where you are!
You'll find loads of info on the right books to work through on this website. We've mainly used CGP, but there are lots of others. You also need to consider if you'd like to work with an external tutor or if your DS will work well with you.
From a personal point of view, my top three tips for you are:
1. Vocab, vocab, vocab
Get him reading LOADS and check all the time he understands the words. There seems to be an infinite amount of vocab in the CEM 11+, and much of it is really challenging. There are also lists on this website which can give you a starting point. Keep a list of the words he doesn't know, and get him to look up synonyms and antonyms for them on the computer. I also made up flash cards in bright colours, and then made up silly games like snap.
2. Check all the basics are in place - esp being really quick and accurate at times tables. It is sometimes surprising where the gaps are. Make sure he understands all the maths he is doing at school during Y5, and try to extend his learning for each topic wherever you can. Then, address any areas of weakness, until he can tackle questions without thinking about it. For example, I realised that my DS couldn't read timetables, so I printed some off the internet and made up questions for him until he was confident.
3. Above all, prioritise the happiness of your DS above all else and don't lose heart if you hit a plateau! They are still so young, and go through highs and lows of learning. Do everything you can to keep the pressure off - little and often has worked well for my DS, and you'll know better than anyone what will work for your DS. Keep him well-motivated (e.g. star charts, treats) and praise his efforts whenever you can.
And finally, seek solace and advice from this website, where you are among like-minded souls who are going through the same thing as you. Good luck!