Telling my son about his dad

So my ex partner was very abusive, when i found out i was pregnant i finially decided to get the police involved and leave as he was no longer hurting just me anymore. I started dating my current partner (who ive known for 6 years) during my pregnancy. My baby is now two months old and my partner loves him as his own. I was wondering when and how we should explain this to him without is causing him any emotional damage.