How do I keep a little pinkie mouse alive?

Long story short I ended up purchasing a little pinkie mouse from the pet store. I have had it for about 4 days now.....I am feeding it kitten formula and rubbing it's stomach to help it go potty...which it is doing. Anything else I should be doing? Just felt so sorry for the little thing....there were no adult mice with it in the cage. What else should I do to keep it alive?