the particular way in which he has trouble walking indicates neurological damage; a brain tumor heads the list of possibilities.

I lost the first cat I raised from a kitten to what the vet first thought was a brain tumor. It was something more obscure, but still untreatable, so the result was the same. I had her put down when her eyes stopped looking confused and started looking afraid; I couldn't justify keeping her alive after that, no matter how much I didn't want to let her go.

((((hugs))))

It hurts to lose them, but sometimes it's the only right thing to do.