What to do about this guy

You are 19 years old and he has a 3 year old daughter.  You said yourself that it is complicated.  You know it's complicated.  Do you still want to be a stepmother to a 3 year old.  It will reallly really really get complicated if you becomes the wife of this man.  Do not mind what the ex will think.  Decide for yourself if you are ready for a complicated life at a young age.

    You need to find a man who does not have kids below 18,  I was a stepmom before and it was a catastrophe.  Kids could create problems in a relationship,  there are exceptions I know.  I hope you will make the right decision for yourself.

Disregard the ex, first off...she's his ex, and therefore has NO call to say who he can and can't date.  If she hates you it's just because she's jealous that you make him happy and she couldnt.  Second, don't hide your feelings from him.  Maybe don't come right out and say you want more, but give him the indication and see where he takes it.  If he bails when you indicate that you want more than just a "casual" relationship then it means he was never going to get serious about you in the first place and you're MUCH better off finding that out now than waiting around 5 years for him to make the first move.

The best way to handle this is to simply be honest about your feelngs and how they have changed.  If he really cares for you he will understand.  Just explain that as you have grown closer to him you have developed a desire to become more serious.  And as far as his ex - if they are not together it shouldn't matter to her if you and him are serious or not. 

I agree with everyone who answered this question.  You could just say, one night when you are alone and maybe watching a romantic movie or even a ball game, "you know ever since we have been together, you make me so happy and are fun to be with.  I think I am starting to like you more then just friends".  Or in your own words.  Don't be to pushy in case he might not feel the same way. Just be casual about it.  Forget about his ex. She's out of the picture.  His daughter is another story.  If you are both mature people you can figure a way to include her in your life and be happy.  Good Luck.