Those are VERY high scores. With those scores I will be grinning. Over 127 means your son is in the top 4% of his age group in the country.
Anything above 120 is a definate go (according to what I have been told) but like anything the test are not conclusive. 110 -120 is give it a try and under that means you need to put quite a bit of work
This explains more on cats percentiles
http://220.127.116.11/search?q=cache:L5 ... clnk&gl=uk
Saying that I have seen some forum members whose kids had pretty low scores below 110 and they worked hard in those areas and these kids went on to achieve really HIGH scores (almost full marks) in the 11+ . So that shows even though cats can be an indicator, its not a guarantee to anything.
In short you may not need much preparation but there is a certain amount you still have to do to get the techniques. There is still time until september if you start working with you DC now. As for your scores, those are very high but if you are aiming for superselectives then bear in mind your child will be competing with those of similar intelligence or higher or those highly tutored or coached for the exam
Judd took boys from about 415 while skinners took from about 410. I am not sure if you do know that 420 does not mean they got everything right.
Roughly the child need to have scored around 75% to 80% to get the full marks of 120