Maybe you could put him in something snuggly to set it and him into the seat with, and buckle him up around it... that's all I've got.
And I don't live in Fairbanks (yet!), but I hope you can figure out something better. And hey, maybe you'll keep doing the carseat back and forth thing, and your back and hips will just get stronger and stronger, and then it won't be a problem anymore.
ETA: Just saw how young your baby is - Just keep on carrying him around everywhere with everything you do in a carrier of whatever sort (Ergo, Storch and Didymos were my favorites of the 10 or so we tried), and your back and hips will most definitely get stronger with time. My back is still just ridiculously toned from carrying my dd around since she was born. She's too big for carriers (though she still hops in the Ergo now and then), but I carry her in my arms often, so now my back and arms are pretty crazy awesome.
Anyway, just wanted you to know you will get in wonderful shape as time goes by, so just try to feel strong and enjoy carrying the carseat back and forth for now... as he gets older and bigger, you'll be getting stronger to be able to carry a bigger babe and the seat!
That's just how I think it'll be anyway. Good luck!