If you changed the food, maybe that is one of the reasons it's happening, have you taken him to the vet, maybe there is something wrong.
Also I personally don't think that yelling & putting their noses close to the poo helps at all, if anyting it just scares them (expecially a siberian, they are so unlike any dog I've ever had).
I know I can't yell at Roxey, it just scares her.
I hope you can find an answer without having to leave him outside all night