My cat found a mouse in my garage!

Hello, yesterday afternoon my cat was found trying to play with this little mouse in my garage, my parents took him inside because he was so small and wanted to get him warmed up. They gave him some bedding and a little dish of water, later on I also sprinkled around some hamster food because that’s all I had on me but by that point he was in the bedding and I couldn’t see him. This morning I finally saw him for myself and realized how tiny he is. Is there anything I can do to help him? I don’t even know about how old he is, and I don’t wanna end up killing him bc I didn’t have the right stuff!