I don't have health care knowledge for a mouse other than the basics, which Vlad shared.
---Keep in a draft free container.
---Provide a bit of warmth, like a hot water bottle under one corner of the cage so he doesn't have to expend healing calories trying to stay warm. (Don't let it get too hot, and only do one corner, that way if he gets too hot, he can get away from the heat source)
---Keep him in a quiet dark location to minimize stress and disturb him unnecessarily as little as possible. Mice are extremely vulnerable to stress.
---if he's eating and drinking, make sure he stays hydrated. I'm not sure about the applesauce, because I know it can be a diuretic with some animals? Perhaps some unflavored pedialyte to keep his electrolytes balanced? Babyfood would be a good choice. Maybe some dried rice babyfood you can constitute at home as needed.
---Is Asuna a female? If so, she may be a comfort to him while he's sick. If she's female, I'm assuming you got them as a breeding pair.
If Asuna is a male, you'd want to remove him because males inherently don't get along, and even if they are young enough to still be friendly, it would likely still be stressful while he's sick, unless it's very clear that they are snuggly and peaceful for the time being.
That's all I can think of though. There isn't a whole lot of info out there on mice care other than 'get em to a vet'.