My son started at SHSB in September and we are delighted with the school. He travels from Billericay station every day and the journey is absolutely fine. There are lots of children travelling to both the boys and the girls schools as well as other schools in Southend. There are plenty of children who travel in from further afield and change at Shenfield. My son certainly enjoys the social aspect of the journey.
If he walked to the local comp he would probably only save 15-20 mins each way and we feel the benefits of the school far outweigh the slightly longer day.
It may be worth you doing the journey yourself on a school day at the time your son would travel and see for yourself if it would be right for him.
Hope this helps