Having worked for both of them, if you bought it from petco or petsmart, i would not put absolute faith in the date he last fed, or that its eating f/t. I don't mean to imply they all lie and cheat, but only one manager in ten years (other than me) had a strict no-cheating policy.
Before anything else, I would make sure the thawed mouse (rat hoppers are better) is warm enough, but not cooked, or the snake will snub it. I prefer to use hot tapwater (hot as it gets) and soak the rat about 20 minutes, then refill with hottest tapwater for about 20 seconds. The first soak thaws it out, and the second warms it to the core. This warmth is only good for about 5 minutes, then you'll need to repeat the second warming if it isn't taken. I also keep a towel next to the thaw tub, so when i pull the rat, i can quickly pad it semi-dry as i bring it to the snake.