Cat w/severe behavioral problems

Hi-

I'm new here and I know this is a highly emotional subject, and I
apologize, but my cat, Nutterbutter, has undergone a severe personality
change within the last few months.

He's 14 years old, and though he's been a relatively loving cat over
the years, as of late he has become _very_ aggressive toward others,
hissing and refusing to be petted. His appetite has remained the same,
and there don't appear to be any other physical problems.

I am very worried.

The personality change is so severe, I'm seriously thinking he may have
a brain tumour. I am very concerned, and as our home enviornment has
remained steady, I cannot understand why he is behaving this way.

I am afraid he will harm one of our children.

I am going to take him to the vet today, but I am so worried they will
tell me has to be put down.

My heart would be broken.

Any else have a problem like this?

~Thanks~