I honestly don't think the chicken broth will have any effect.

If he won't eat on his own, keep assist feeding once every 5 days. If you can, have that person teach you how to assist feed so you can do it yourself.

If you haven't tried already, try heating the prey item with a hair dryer right before offering it. The warmer it is, the better they can see it. Also, the hair dryer makes it nice and smelly.