Starve Acre by Michael, Hurley, Andrew

The worst thing possible has happened. Richard and Juliette Willoughby's son, Ewan, has died suddenly at the age of five. Starve Acre, their house by the moors, was to be full of life, but is now a haunted place. Juliette, convinced Ewan still lives there in some form, seeks the help of the Beacons, a seemingly benevolent group of occultists. Richard, to try and keep the boy out of his mind, has turned his attention to the field opposite the house, where he patiently digs the barren dirt in search of a legendary oak tree.