Advice for a cat who won’t draw boundaries?

I have a female spayed 4 month old chihuahua-Shih Tzu-pug mix. We have 2 cats, 10 and 12 years old. The less friendly cat has clearly drawn boundaries but the other one – a fat orange floof – seems to just allow the puppy to do bitey face. He will yowl and complain but still lets the dog jump on him. I thought over the past two months he’d get the picture that he needs to have some kind of boundary but he just seems to give up. How do I get the dog to figure out that biting and trying to drag him around requires HIM to stop her? He goes as far as laying on his back and letting her bite… it doesn’t seem great but I also don’t know how to deter her if he doesn’t define his boundary with his front claws. She is relentless with him. Any advice? We have a trainer and puppy k and the only thing that I’ve gotten her to stop with is saying “treat” so she comes to me instead. But then she will just go back to tormenting the cat right afterward.