Recently aggressive cocker spaniel

Question
Hi, i have a 7 year old cocker spaniel and in the last year he has become aggressive at times. He never snapped or growled at anyone ever in the first 6 years we've had him. Over the past year though, he's become unpredictable and while being pet by me [17], my sister [aged 14, who had been afraid of dogs prior to getting ours, my brother [12], parents or anyone else he has seen before he could just suddenly turn and bark viciously and snap at whoever was closest. I thought it was because he felt surrounded or not comfortable but it happened yesterday to my aunt and cousin and my aunt said that something wasn't right mentally with him and that after owning several cocker spaniels, she had seen them act up but never like that and to look out for tumurs?
Also, he's not allowed around the house so he's confined to one room or our big back garden, we have stopped giving him treats while he's indoors because we thought he was snapping because he had food but he's done it since and our dad does hit him when he attacks, would any of these contribute to this behaviour?

Answer
First of all - tell your Dad never to hit or yell - all it does is put more aggression in the room.

A sudden change in behavior should be followed up with a complete medical exam and, in this case, I'd strongly advise a full 6 panel thyroid test.

Confining him to one room is going to make things worse (is this just recently?..I hope so).

So first get him to your vet and know that cockers are very prone to thyroid issues and it can cause inexplicable aggression.  Easy test - easy fix with inexpensive daily meds.  Be sure your vet sends out a full 6 panel test and doesn't just do a snap T4 in the office.

Usually these bloodtests are sent to the local vet college for analysis.