What I say here may not apply in your jurisdiction, so you should double check, but as an AF in an AF/YF relationship, you have no legal responsibility to report anything that the YF tells you, because you are not in an official position of any kind.

This means that you can keep any confidentiality that you like. Personally, I would keep everything confidential, including abuse, BUT encourage him to talk about that abuse with the proper authority who can deal with it, and support him as much as you can. But use discretion in all cases. If his life is in danger for whatever reason, you may want to consider out and out breaking confidentiality. Life over limb, and over relationship...