Your dad is frustrated. It’s ok to be frustrated because in his mind he so badly wants you to not have this interfere with your life. All due respect to your dad but he’s handling it like an idiot though. He needs to stop and stop now. He can either be an ally to you, help you find treatments that aid you and be supportive or he can sit down and shut up. At least sitting down and shutting up wouldn’t make it worse. My dad started in with that at first when I was a kid but I think he soon realized he was better off sitting down a shutting up. I would have much preferred he be an ally but at least he wasn’t building my anxiety anymore. I know confronting anyone as a stutterer is like asking someone to go lift that 2000 pound car over there, but maybe you can talk to your mom or something to tell him to help or get out of the way. Stop making it worse.