sorry to hear that. i don't know how good of an idea it is to cast negative spells while pregnant [just concerned negative karma might effect the child] i do suggest trying to forgive and move on.
i've never been in that situation, but i have a friend who was back in high school. the boy denied her son was his, but once the boy's mother saw her gandson she instantly made her son step up. my friend and her son's father aren't together, but he see's his son every other weekend. [child supports kind of non-existent but still] i don't know if your situation's the same though. if my friends situation taught me anything it's for the first few months you'll be hurt and angry, but eventually you'll realize this guy isn't worth the time of day. i do suggest either a DNA test, or a court order so he does take responsibility [my friend did it, though since they were minors it was null once she turned 18] also, if you keep his name off the birth certificate he has no parental rights [again, she did that, hence the DNA and court order] just options.
now, in terms of a revenge curse/hex, i do suggest waiting a little while, meditating, and trying to find an alternative. i don't know the full story, but if it turns out you are in the wrong [theoretically it can happen] and you curse/hex someone who doesn't deserve it, it can backfire badly on you. if you still feel it's justified, try a simple 'messed with a witch' chant.
it's effective, but it should just be enough so he gets punished for leaving, but not so much as you'll feel major negative consequences.