It's hard to say for sure, but generally an "office hearing" means that the DA or City Attorney is willing to offer some alternative to criminal prosecution in a case. They might be willing to offer your former boyfriend counseling or batterer intervention classes, or something similar, instead of trying him in criminal court. So in some ways you're right it is more like a mediation than a prosecution.
HOWEVER, I strongly suggest you contact the DA or CA on the case. Look on your notice of the Office Hearing, there should be some contact information. Let them know what happened, urge them to file charges and prosecute if you want your ex prosecuted. If you have evidence to support what happened, let the DA or CA know. If your son is willing to testify, let them know that too. The DA or CA can still change their mind, usually, before their office hearing or depending on what happens at the office hearing.
If you are fearful of attending the office hearing because your ex will be there, let the DA or CA know that too.
If you are fearful in general of your ex, as he was arrested for this last act of Domestic Violence, and you said it's not the first, you may want to obtain a restraining order against him. You can also ask that the restraining order protect your 6 month old and your 16 year old as well. Remember your ex will have some rights to see his 6 month old, but you also have the right to be safe and free from harm.
I agree with the previous attorney's assessment. It might be worthwhile for you to contact a battered women's shelter or the victim/witness office at the courthouse to see if they will assist you in obtaining a temporary restraining order if you are in fear of your safety and that of your son.
You can always put a statement in writing and ask the police detective to forward it to the DA's Office. I would recommend you contact someone at Victim/Witness to provide you with information on the best way to proceed in your county.