Please don't do it.
Any advantage you think that he will gain by attending a grammar school, any grammar school, will be far outweighed by the stress and time spent travelling forty miles each way every day, five days a week, mine months of the year. Travelling with the dominant flow of traffic on the A12 / A130 / A127 / local roads around the school, at the time of day that he will need to in order to get there on time every day - an hour from Colchester to WHSB or SHSB for an 8.30 start? Seriously?
By train, by the time you have factored in the change at Shenfield and the walk from the station at this end, SHSB would be a better bet than Westcliff, as PrIttlewell station is on the same line as Shenfield. To get to Westcliff, you would have to catch a bus from Southend Victoria, or from Rayleigh - and that option involves a walk up from the bus stop on top.
Much as we like SHSB, I wouldn't even have considered it an option if we lived forty miles away instead of two.
There are some nice houses on the market in the area at the moment, though, if you really, really want your DS to come to school in the Borough of Southend on Sea. I think you have until sometime in early January to move, in order to be classed as in catchment.
Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx