Son asking about his dad..

My son is 2 now and he's starting to ask where his daddy is .. long story short I had to leave his dad while I was pregnant be ause he was becoming extremely abusive and was son had drugs. I'd gotten a restraining order and I'm still proteted to this day.

My son never asked about his dad until recently when I had to put him into daycare. He sees other kids daddies and wonders why he only has a mommy...

How can I explain this to him? I'm so lost