Why is my ferret docile, and non-responsive when my new puppy jumps all over him?

I've had my ferret swiper for a little bit over a year now...i believe he he is about 15 months old. He is very playful and active especially around other animals (he loves to chase her cats and dogs). However, I just bought a 2 month old female jack russell terrier, and when we try to introduce the two, my ferret either freezes in his tracks and allows himself to be used as a rag-doll, or he hides somewhere. Before I bought the Jack Russel, my vet told me that introducing them would be fine, and that the ferret could hold his own, but when the two animals are together swiper is always petrified. Is it always going to be this way? or is he just not used to having another animal living with us? I am afraid of the terrier killing my ferret...