Personally I would make it something he knows from day 1. He is going to notice at a young age that other people have fathers and he doesn't so I think it would be a lot easier if it is something out in the open and easily talked about.

Good luck!

Yea, something really simple for him to understand... (some people grow up living with 1 parent, some with 2). And leave the details for when hes older.

I think that's right. Even babies in the womb can be talked to. They will understand. And they are prepared to respond to those who will ask them when they grow up why they don't have a father.