Good for you for doing the right thing.
It took courage to come clean, and it will be hard for a while but you've saved a lot of heartache and pain from happening later on.
As for who the father is - it depends on if you used protection with the other guy. If you didn't use protection, then its still possibly that some of his semen could have gone inside you even if he didn't ejaculate. He also could have ejaculated without telling you (although you'd be likely to notice.)
It also depends on when the sex happened with that other guy. Can you try to refresh your memory somehow to figure out when it was? Even if you connect the memory to other events- what else was happening in your life close to when you slept with him? Sometimes I look over facebook messages to figure out dates for things. Maybe you could try that?