I’m trying to teach my dog to respond to backward leash pressure and he always sits and doesn’t back up. Do you have any advice?

Usually when the dog sits it's because the trainer is accidentally putting the pressure slightly up, which makes the dog respond by sitting. it also may be that you're asking for too much movement back at first. I start with a very slight movement or weight shift back and mark/reward that. You can build on that as the dog understands. Don't expect much movement at first. Tiny increments of movement back should be rewarded and the pressure should be released immediately. Keeping the pressure on without a fast release will also result in a sit.

Make sure to review the angle of the leash pressure and make sure it's being applied parallel to the dog's body as any pressure upwards will almost always result in a sit.