He has no idea what he's going to do. It's like when a parent tells their kid to stop something and then starts counting. If he knew what to do he would have already done it. Maybe he'll try small claims court again, the filing fees are about at his level. Or he'll figure something else out. But his hubris makes him think just his "legal" threat will be as scary as if it were from a real lawyer.
A DMCA takedown notice? That doesn't require any damages, it's just a demand for a hosting site to remove copyrighted material. If they subsequently remove the material, they're safe. If not, they no longer have safe harbor and the owner\operator can be sued like anyone else.
If he were actually to go to federal court suing someone for infringement he would have to go after the profits they made (not applicable), actual losses to him (probably about zero, since no one is willing to pay money for his book so he would have had the same sales) or statutory damages. The latter would be most useful but also would require him to have registered the copyright of his work before or soon after he published it (too late now).
