I would bring him in to raise to the chrysalis stage and then keep in a cool (not freezing) place so it won't come out till next summer. I know more will comment. Make sure he gets plenty of fresh food till this happens.

If you leave him outside, he will pupate and then overwinter where ever he attached, and come out in the spring. Of course there's the chance he will get eaten now, or his chrysalis could be damaged in a bad storm, etc. etc.

If you want to raise him, do what flcajun said. A garage is or shed is a good place to keep a chrysalis, or a screened in porch that has some protection from the wind. What you don't want is to keep it inside your warm house, because it could emerge in the middle of the winter.

My first batch of Black Swallowtails overwintered, and the last chrysalis from this year is likely going to.

Every year I find at least one or two black swallowtail caterpillars still eating outside even after our first frost. Once the sun comes out they warm up and continue about their business :)
If I don't bring them into "protective custody" to raise indoors, then I'll put bridal netting over the plant (so the little guy doesn't wander off), held in place with rocks around the base, and let it finish eating and form its chrysalis on a plant stem.

~ Judy
www.ButterflyNature.com

Thanks all!
I think I'll leave him outside - I don't cut back my butterfly garden in fall so the odds are in his favor. I appreciate the info on how to raise him myself - but I'd feel bad if he didn't make it. Thank you for the info - I had no idea how they overwintered.
Lucy

I have two BST cats on the bronze fennel. I think one is about to pupate. I checked on him several times today. They were munching away so I put up some tall edging fence and some netting that doesn't touch the plants. The bluebirds are everywhere and they will surely find them without the netting. It was in the 90s here yesterday, but was 68 today so cooler weather is here.