Parent the pants to the hip. Make sure the prop is selected then in the object menu choose change parent and from the new popup choose Jeremy's hip. This won't cause the jeans to follow the bends of the figure but they will stay with him as you move him around. And you can save them with him. But you will still need to run a simulation for it to match a pose as shown in the three images.

You'll also notice that the jeans have moved in the hierarchy menu to being listed under the hip rather than as a separate prop.

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