how can I appeal his ban?
You can email Fizzer at firstname.lastname@example.org. Make sure you make your case succinctly and obsequiously, because he has been known to increase bans or issue new ones in response to ban appeals. (He's Judge Judy and executioner- so it's a system that responds better to flattery than reason.)
If you have any way to prove that Cazza is not an alternate account and that you, Robert, and Cazza are distinct people, you should just focus on providing that proof. If Cazza was detected to be an alt account, that means Cazza was determined to be playing on the same network and likely the same device as the alleged main account.
You should probably have Cazza email on his own behalf because Fizzer is also typically unwilling to discuss players' bans/suspensions with anyone other than the banned/suspended player.
If you are telling the truth and Cazza was banned in error, good luck correcting that error. I also hope you, Robert, and Cazza choose to behave more respectfully in this community going forward.
Edited 1/23/2022 12:57:40