Rest assured, you have not left it too late, and September/October is actually the optimum time for preparation for the 11+ to be taken the following year. I tutored my own son and it really is quite straight forward, though the relationship thing can sometimes make it hard going!! Get yourself off to Waterstone and check out the 11 plus section there. You could start with the Alpha series as these are not too difficult and can help to build some confidence. Make sure you you practise verbal reasoning, english and maths to start with then move onto the non verbal reasoning a bit later.then you might want to try some of the Bonds series for maths and english plus there 10 minute tests for verbal reasoning.
The important thing is to get a variety of practise materials as no one on their own quite 'does the job'. Later, say May/June, you could move onto the Bright Sparks series for all the relevant areas. Don't panic if your child can't do something/ or is weak in area, just see this as a positive identification for improvment areas and work on them with him.
FM is the real expert here and I'm sure she will be along with some very useful tips.Good luck, and I'm sure you will be as good as any tutor, just be prepared for the moans and groans he wouldn't display in front of a teacher/tutor...but it really is worth the effort/trial and tribulations in the end!