Firstly, there is no official 12+ in Essex. Basically, what happens is that each school holds a waiting list until the end of year 7. After this you can write to each individual school and ask for your son to be tested if and when a place becomes available in his year group. In practice, it is a bit of a lottery because, usually only one place comes up at any one time and I think all the schools have a policy of each child only being tested once for each particular year.
You will not know how many children have applied for a place in that year group and therefore your son could be taking the test with only himself and another applicant or, if unlucky, there could be many applicants.
The school concerned will offer the place to the child with the highest entrance marks in the test but if no children do well in the test and no applicants fit within the "ability parameters" for the school, then the school does not have to fill the year group with the extra child (this is the law as it stands with grammar schools.
The hard thing is that your son could be taking the test in a situation where one child is "super smart", possibly a child who is moving into the area from another grammar school, or, it could be a case that the majority of applicants are fairly average, whatever the circumstances, there will only be one chance each year.
The format of the tests depends on the school. Most test in English, maths and vr, in a format not unlike the 11+ others, depending on their specialism, will test other areas, such as French. To be sure what is required you should ask the school/s concerned.
It is possible that you could write to school/s at the beginning of the year when your son is in year 8 and a place may almost immediately become available, as children move out of the area, cannot cope with the work or whatever reason. However, you may have a much longer wait, it is luck of the draw.