Hmmm...I kinda like that idea about letting Jack out by himself, without Bella there. Just see what happens.

You said they do play a lot, and Bella could have hurt Jack totally by accident!

Whatever happened, he does have to be trained to go outside as if he had just arrived at your place and had never been outside.

The only non-vet thing I can think of is Kennel Drops; they go in the dogs' water.

Prayers that Jack is all right soon.