Hi, I tutor for the Essex 11+ and can hopefully alleviate your worries. If you have further questions, and I'm sure you will, feel free to post. I also wrote a wiki that is on the site, but I am struggling with the link.
1. There is still plenty of time.
2. Anyone can enter their child for the Essex 11+, which area are you specifically. Southend also has grammar schools that use the same entrance test. Redbridge has a different system.
3. The subjects covered are maths, English comprehension and VR. VR is covered very well on this site, but the exam is time critical so I would recommend watching the videos before you start tutoring your son. In this way you will be able to use all the hints and techniques that experienced tutors have mastered. The maths and English papers are written by the grammar schools and you can buy the papers from the last two years from the CSSE
www.csse.org.uk/ VR is set by GL Assessment (NFER Nelson). There are 21 types and 12 of these types will be examined on the day. Regarding suitable material; Bond Assessment work books are very good for English, concentrating on the comprehension sections. The grammar section is not examined in the 11+ anymore. For maths Bond are OK, but there is a lack of problem solving. I would recommend looking at the primary maths challenge book
www.m-a.org.uk/jsp/index.jsp?lnk=250 The QCA gifted and talented problems are also useful
nationalstrategies.standards.dcsf.gov.uk/node/852604. It is very difficult to get in to the Chelmsford and Colchester grammar schools. Many children do not manage. The Southend schools are significantly easier. The Chelmsford and Colchester schools do not have a catchment area, which means they take the highest scorers in the exam; 75% is a borderline candidate (top 2%). Southend, within borough, take children with an average score of approximately 55% (top 25%). Out of catchment children can also enter for the Southend schools but their scores generally need to be around 65%. There is no passmark for any of the schools as such, but Southend has a minimum cutoff of 303 from a complicated standardisation process.
5. On the plus side your son is already scoring 5s. Although the SATs are very different from the 11+ the indication of 5s in year 5 is always positive.
6. For your application you will need to complete a CAF/SAF, which is the general secondary application form. There is also a SIF for the grammar schools. Both forms must be completed.
7. Vocabulary can often be a problem, the comprehension text is often a 19th century novel by authors such as Dickens or Hardy.