It's pretty much up to him because if he did, he could easily find a lawyer who would take it on contingency. The CDA probably immunizes people who did nothing but retweet the original, but it wouldn't for anything they went and did offline, or anything libelous they said themselves.
Since it was a general harassment scheme they were all participating in together, and calling someone a Nazi is clearly defamatory, they'd all be jointly and severally liable. This means counsel could cast a wide net and sue basically everyone he can find, and then every one of them would be liable for the full damages. So if one guy out there has a few million, they could collect it all from that one guy. Then it would be up to that one guy to sue everyone else to try to get back some of his money.