I think the problem is the poster can easily change IP addresses (cell phone? Definitely with VPN), it would be almost impossible to stop it.
Only thing I can think of is - just dox the guy. If there were multiple posts from the same IP address (likely earlier posts), that's probably his real location. Just dox the guy and let the internet do the rest.
Guy is obviously a ***** with some issues but you can’t find out who someone is by their IP.