English Art/Literary Poetry & Literature

Context:
If he’d had spurs, Moist would have spurred Boris on at this point, and would probably have been thrown, trampled and eaten for his pains. Instead, he lay low over the horse’s back and tried not to think about what *this ride was doing to his kidneys*.
More context, please click:
http://pkhedar.uiwap.com/Ebooks.txt/Pratchett-Discworld_33-G...
--quoted from Going Postal (2004, fantasy fiction) by Terry Pratchett

As the text says, Moist was riding a tough horse, Boris, without saddle. The author used “he/his” to refer to both Moist and Boris, and I nearly know nothing about horse riding, so I cannot determine whether “his kidneys” refers to Moist’s or Boris’s. What I know is that when you sit (or put a saddle, but in this case, no saddle) improperly on the back of a horse, you may hurt the horse’s kidneys, but Moist was a good rider, who would avoid this situation if he could. On the other hand, Moist’s kidneys seem may be hurt by continuous vibrating during the uncomfortable riding.
Thank you!