Confused about a sentence

It sounds like Person B has pinned her hopes on Person A.

How do you use 'Hope ' in a sentence where you want the person to know that someone is in hope that he would help him. Is this sentence correct- He is holding on hope against you.

hold out hope is an idiom: TFD

To maintain hope that something will happen or be the case, especially when it does not seem likely. Often used in negative constructions, and often used with modifiers like "much," "any," "little," etc., before the word "hope."

Your sentence grammatically would be:

He is holding out hope that you would help him.

There are many options for you. Here a just a few:

John is holding out hope that you will help him.

John is holding onto hopes that you will help him.

John is keeping his hopes up that you will help him.

John is still hoping that you will help him.

John is still hopeful that you will help him.