Legally this is just the same as what Westboro does. The high level of offensiveness some people find in it doesn't make it illegal. And since he is not her healtcare professional and her attorney isn't suing for slander he hasn't done anything legally wrong. Billboards are not coverd under harssing communications and in this case since the communication is not directly to her so I don't see much legal room for her to fight him.
I'm 100% against abortion for ANY reason and think it is completely unfair that women can murder their children without the father having a say so. So while his message is something I agree with, I wonder what his motives are. If his motives are to bring pain to her, he is in the wrong even if the abortion pained him. Revenge is the Lord's, so sayth the Good Book. I read this on foxnews earlier and one of the comments was where someone checked this guy out on facebook. He belonged, according to this one comment, to Facebook groups about witch craft including one that claims Jesus was a witch. I guess I would have to better understand his motives to decide if I supported his actions.