Where abouts in Idaho do you live? I'm over in Spokane, if you're near the area I can suggest some local places to get mice for him. None that I know of sell appropriately sized rats frozen, and the ones that sell live only have the rats stocked occasionally (they sell quick, either as pets or as food) so you might be limited to mice. You can order frozen prekilled rats online though. He's likely very stressed from his roaming adventure outside in the cold, and stressed BPs are notorious for being finicky eaters, so you probably want to give him a couple days to get settled in after you get his set up squared away. Then you can start by offering him a prey item that's about the thickness of his body. Oh, get a pair of feeding tongs or a hemostat to feed him with, as you don't want to put your hand into a snake's cage when feeding where he could mistake it as part of the mouse. If you have the cash after everything else, you should pick up a heat gun too. You'll use it to double check your temps in the enclosure, and you can use it like I do to check to temp of a heated up f/t food item.

You can try offering live or frozen/thawed, but there is a proper way for each. I would personally try live for the first couple of feedings to make sure he's got a few good meals in him, then try making the switch to f/t. If you get live, make sure you stand by and watch so you can intervene if the mouse or rat decides to take a bite out of your snake. If he doesn't act interested, remove the prey, DO NOT leave it in with the snake. That's a sure fire way to come back to a chewed up snake

When you want to try f/t let it defrost slowly, then heat it up to live temps when you're ready to feed. I way I do it is I put mine in a cup of cool water in the morning, then run it under the hot tap water in the evening before I feed them. Live rats and mice have a body heat of around 101F. My hot water comes out of the tap at around 115-120F, I leave my rat pups under it long enough for them to temp at around 109F with the temp gun, and then by the time I get them up stairs and ready for eating they've cooled off another couple of degrees so their just a little warmer. Others will use a hair dryer or a heat lamp. Some forum members swear by heating the rats/mice up in the room so the snake can small it and get hungry. You might also find you need to wiggle the prey around at the end of the tongs to make it do the "zombie mouse dance" to entice a snake to eat.