“were acting as de facto government agents”
This was always going to be a tough case to make, even if he brought it before a friendlier court.
Probably a much easier lawsuit to win would be to argue that, since there was clearly collusion between facebook, twitter and google/youtube to ban him and other conservatives in coordination, they are engaging in anti-competitive business practices. That wouldn’t necessarily get him restored to the platforms, but it could be devastating for the social media giants.
What about a defamation lawsuit? Surely Twitter has repeatedly accused Trump of being a liar. Twitter has done so by publicly stating that Trump has violated its policies on spreading disinformation. So why not sue on the basis that many of these instances involved provably true statements by Trump; thus the claim that they were disinformation has defamed him. Being a public figure means he would probably not win such a case but perhaps he could get lucky with a judge and survive to the discovery stage.