Congratulations on your new addition, and welcome!
Do you have proper heat and humidity in the cage? Make sure to not mess with/handle him until he's eaten at least once.

If I were you I'd give him one live at first, just so he's eaten something, and you know he's properly settled.
When you heat up the mouse, make sure it's really warm, like if you held it against your cheek it's warmer. Get some tongs and hold the mouse by the scruff, wiggling it gently in front of the snake to imitate it being alive. If he doesn't take it after a few minutes, leave it in the enclosure overnight, and remove in the morning if uneaten.