Hi Peter,
I think you might have conflated two species

There is one species called Hermann's tortoise and another species called Horsfield, but not one called Horsman. Thanks so much for the photo -- he (or she) is a handsome little Horsfield tortoise and here is a link to a good care sheet for Horsfields:
https://www.tortoise-protection-group.o ... 014New.pdf
It sounds like you've created a really nice outdoor area for him, but I think you are right to put him somewhere cool that is protected from predators. It actually needs to be fairly cold, because if the temperature reaches 10C then they start using up body fat, so ideally you want a temperature of between about 2C and 8C. Also, he should really go into hibernation with an empty stomach (which he seems to have) and a full bladder, so that he doesn't dehydrate in hibernation. If you could manage to give him a bath in tepid to cool water (for 10 - 15 minutes, in a container he can't see out of, like an old washing up bowl) before you put him into hibernation then he might take on a bit of water. If he does have a wee in hibernation then you will need to wake him up.
Do weigh him before you put him into hibernation, and I would recommend hibernating him for no more than about 10 - 12 weeks (although if he loses less than about 3% -4% of his body weight in 12 weeks then you could leave him in for a week or two longer. So because he has gone into hibernation now, that would mean waking him up at the end of January/beginning of February and he will have to have an indoor heated area to go to.
Here is a link to a general article on hibernation, which has some detail as well:
https://www.tortoisetrust.com/post/safe ... r-tortoise
Here are specific directions for preparing for hibernation (which he has sort of done himself, so it is for information). The reason why they need to be deprived of food for between 2 - 4 weeks, depending on weight is because any food left in his stomach can rot and produce noxious gases, but that last bath I mentioned above might also help to move along any food that is left in his system.
https://www.tortoise-protection-group.o ... cedure.pdf
Note that the four week period for winding down is meant for larger tortoises and if your tortoises is smaller then you can divide the four 'weeks' into four units of fewer days (for example, where each 'week' is only 5 days or 6 days)
Here is a guide to box hibernation:
https://www.tortoise-protection-group.o ... OV2016.pdf
Here is a guide for fridge hibernation (which given recent fluctuations in temperature outdoors can be safer):
https://www.tortoise-protection-group.o ... Sept16.pdf
And here is a guide to waking from hibernation:
https://www.tortoise-protection-group.o ... 080608.pdf
I'm sorry that this post is so long, but I hope there is some information that you will find useful.
Do let us know how you get on and fingers crossed that all goes smoothly.
Nina