If the pup doesn't get it's mother's milk, it will eventually die in a few days. Rat pups cannot survive when they are that young, especially when their eyes are still closed. I was told this because my Dumeril's boa had a f/t mice problem (wouldn't take it.. 2 months of trying!) and so I gave her a rat pup and she took it that night :]
How were you trying to feed your snake? How I got my boa to eat the rat pup was that I put the pup in the enclosure by the snake, turned off ALL of the lights in the room, and closed the door. Every hour I would check, and it took the boa a few hours to take it (all snakes are different, so youre's may take less time or more time). Good luck!