Do you plan on writing about his thoughts and feelings, things he shared with you about abuse and the effects?
I think lots of men who were abused are afraid our SO will "use" or disclosures against us and why some don't open up.
Is it that important to "use" this relationship in your work? If you care/cared for him "now that your broken up" you would consider keeping his confidence and not use his abuse for personal gain. Even if a name is changed, anybody who knows you two could figure it out.
Consider the fact he wanted you to sign a confidentiality agreement, meaning he's asking you NOT to disclose his story, the one thing all survivors should have total control over.