Affidavit of Sole responsibilty

I'm planning to apply for dependent visa for my son in UK. However, one of the requirement is to provide an affidavit of sole responsibility.

I'm not married to my ex but my son carries his surname. We are no longer in a relationship but occasionaly we communicate when there are certain arrangement concerning the needs of our son.

He knows about my plan to take my son with me in UK and he is willing to cooperate as well.

What are the things that we have to do to comply with the said requirement.