If your BP has been used to eating mice, bear in mind that rats & mice smell (& probably taste) quite different. Sounds like he's hungry but not yet convinced-
several ways to help convince him: thaw it with a mouse & rub it with the mouse to impart some mouse scent onto it, or get some "used" mouse litter & rub the
rat with that...either way, it should help. You can also feed a smaller mouse first & then follow it with a smaller-than-needed rat: not all snakes will accept 2 items
at one sitting, but if they do, it's another way to get them to try the "new" kind of rodent. Just make sure that the 2 together aren't much bigger than the one item
he'd have had for his meal. Many snakes are pumped up enough after eating one to eat them both, but it's not a guarantee, just another thing to try.
Did you heat the rat just prior to offering? Many here find success using a blow-dryer, especially on the rodent's head (where your snake should target). BPs are
snakes that especially seem to care if their f/t prey has lifelike warmth...they use heat sensing pits to identify prey.