Hi Annika, and welcome to The Tortoise Table! Congratulations on welcoming this little Horsfield into your family. I'm sorry to hear he's been unwell, but it sounds like you had good service from the vet and that your tortoise is on the way to recovery.
This time of year, tortoises are thinking about hibernation, and Horsfields -- even babies -- have a strong instinct to hibernate. You don't want to hibernate him this year, as you haven't had him long enough, and he hasn't been well, so now you need to convince him that it is a bright summer's day, in order to get him out of his hibernation mode.
Can you describe your set-up? And if you can send in some photos of it, that would be a great help, as we might just be able to suggest some tweaks to get it perfect.
One of the most important things to get right, and encourage him to eat, are the temperatures in his enclosure. I hope you have a tortoise table rather than a vivarium, but just let us know what you do have and we'll try to help. You want to aim for a temperature of around 30C directly under his heat source, and at the height of his shell (a thermometer mounted on a nearby wall will give you an artificially low temperature, so you do need to measure it in the right place). At the cool end of his enclosure you want the temperature to be about 20C. There shouldn't need to be additional heat at night, but if the temperature drops below about 14C or 15C at night then we can suggest some supplemental heating for you. And try to make the light as bright as possible.
If he isn't waking up in the mornings then you should wake him up and put him under the light to warm up. If he goes back to bed, then put him back under the light again, and do this one or two more times (and then let him sleep if he wants). After 2 or 3 weeks he might start waking up himself.
Try hand feeding him something like a very thin slice of peeled cucumber -- most tortoises like that. If he likes that, then next time cut up a bit of a leaf of something and put it on the cucumber when you feed him, so that he can't help takng a bite. Add more and more of the leaf and less and less of the cucumber until he is eating green leaves. If he likes cucumber (which isn't a particularly nourishing food), then squeeze some of the juice onto the food you want him to eat, as that might tempt him. What calcium/vitamin supplement are you using? If it is Nutrobal that might be a problem as many tortoises don't like the taste of it. Here's a link to our articles on Overwintering
https://www.thetortoisetable.org.uk/tor ... 77Hi7OnyM8 and here is a link to our article in introducing a better diet, which might provide a few tips
https://www.thetortoisetable.org.uk/tor ... 77HwLOnyM8
If he isn't eating, do make sure he is well hydrated by bathing him every day or two in nice warm water (the temperature you would bathe a baby), in a container he can't see out of, and the water should come just up to where his top shell meets his bottom shell (or his chin).
I hope this is a start, but please come back with any more questions and do send some photos if you can.
You might alrready have a good care sheet, but in case you don't, here is a link to one:
https://www.tortoise-protection-group.o ... 014New.pdf
Nina