Hi there,
Some thoughts ...
I wonder if any horses were allowed to be imported .... aftterall our AIF personnel were not permitted to bring their horses home .... significant bio security issues, etc.
Could the Clydesdales have been shipped from interstate rather than from Britain. If so your chap may have been quite young when migrating from Britain.
If you chap was in Britain 1915, and born 1899, would he have been called up by Britain to serve in WWI ? Australia's AIF were all volunteers.
JM