Rodger, an edition of The Son of God was published by JAMES CARTER in 1896. This was a full reprint taken from the original papers that appeared in the The Present Testimony. It was edited by H. A. Hammond of Sundridge, Bournemouth, and in his notes he references extracts from John Owen. Prof. Gribben's researches may well have thrown up others. The Chapter Two Book Depot, London, secondhand department may well have copies of this edition available, as they have had In the past. It remains a good source for secondhand books.
We may conjecture why many had been asking for this book "at the present, time" [i.e. 1896].
It is sometimes alluded to that this book was written specifically to counter certain teachings about the Sonship of Christ that were cropping up. Is someone able to contextualize this book for us (i.e., who was involved, where and when, what the particular teachings were)?
WJH gives his guess on who JGB was opposing: https://www.brethrenarchive.org/manuscripts/other-letters-of-w-j-hocking/31st-october-1932/