It will be interesting, however it's pretty clear to me that the suit is without merit. I sympathize with the father but I think his anger is misplaced.
Terrorists do not organize on social media, that would be the equivalent of emailing their planning directly to law enforcement / intelligence apparatus.
Further, if an online service sets a policy of policing any content then they're obligated to do so for all content. As far as I know Google, Facebook and Twitter all follow the line of disallowing hate speech and generally speech that is illegal such as child pornography.
Beyond that they not only cross a significant moral line if they restricted what views they permitted to be aired, they also start to take on legal responsibility. Again, if you edit any content, you take responsibility for editing all content and set yourself up for huge liabilities - this was established in the early days of the internet.