I have been through the experience in Essex on several occasions so I have become fairly expert in the field.
Firstly, Colchester Grammar and KEGS (Chelmsford). Both of these schools currently operate a "first choice" policy. This means that you need to put one of them down as a first choice to be considered at all.
Currently, this does not mean that you lose the chance of a place at Westcliff or Southend or, indeed, your local comprehensive unless your local comprehensive also operates a first choice policy (most don't but Beauchamps in Wickford, I believe, do).
Your son will need to gain a considerable score to obtain entry to either Colchester or KEGS. I believe that 85% in each discipline is a reasonable bet for entry but obviously it differs from year to year.
Now that the system for application has changed for some reason the Consortium for Selective Schools has changed the method of giving results. This year, it was impossible to work out where a child came in the order of merit and equally difficult to figure out if they should have been offered a selective place or not.
The most difficult test (in my opinion) is the English. The test is really not set at the standard of an average 10/11 year old but would challenge the average 14 year old. Again, this is just my opinion.
To obtain a place at KEGS or Colchester your son will need to achieve a score around the 500th position on the order of merit. Uusually, there are more than 4000 well prepared candidates so it is a **** of a quest. The 500th position I give you as an average. It can range considerably between 400 and 600 I believe.
Southend and Westcliff operate a different policy. A pass mark is set each year below which no child will be admitted. They then look at pupils "in borough" who have scored above the passmark and who have put the schools as a first choice. They then fill the school (to 115 pupils or so) with those children. At the 115 mark each school is said to be "full in catchment). They then look at the order of merit for the whole of Essex. The remaining places are offered out in order of the highest ranked child on the order of merit who has put the school (either) down as a preference. NOTE: Not necessarily a first choice preference. In other words, you can put your son down for KEGS, followed by one or both of the Southend grammars and he will not lose out if he has enough points for a Southend grammar but not enough for KEGS. Usually, your son will require more points for Westcliff than for Southend but there is never any guarantee of this.
The situation is such that, hypothetically speaking, a child in Southend may obtain a grammar school place with 100 marks, where a child outside Southend may need 120.
Anyway, if you require any further information at all, please post again/