Take it to a vet.
Sadly there is unlikely to be a happy outcome to this. I think it may die of shock and/or infection soon.
You cannot keep wild mice as pets, and if they are in the house, they should be classed as an infestation. They carry all sorts of diseases. You may see their droppings and think its easily dealt with, but it's the urine which is the main problem.
You shouldn't class anything unless you know the situation personally. We don't have an "infestation" of field mice, they just pop up from time to time since we live very close to the woods. Personally I've never seen droppings anywhere in our house except by the baby, who is doing very well not that you asked. Thank you for your negative point of view, but my research shows that rescued mice (of any kind) survive if taken care of properly. Optimism rules.
Take it to a vet.
I called our vet that we take our dogs to and they said they would just euthanize him. They literally used the word euthanize. I'd rather take my chances keeping him alive than make the decision to take his life. He's actually doing much better, seems happy every time I locate him in his little nest.
I hope the little chap survives, and you are able to release him back to the wild. My comment was not meant to be negative, just realistic.
Hopefully someone on this forum from your side of The Pond will be in touch. There are quite a few international contributors amongst us.
I called our vet that we take our dogs to and they said they would just euthanize him. They literally used the word euthanize. I'd rather take my chances keeping him alive than make the decision to take his life. He's actually doing much better, seems happy every time I locate him in his little nest.
Okay. I'm in the UK.
Good luck with the whole project.