cat very very sick, HELP!

Hi, I need advice for my sick cat. He's going on 11 years old. He's gotten
thinner, when i pick him up he's much lighter and you can feel his spine
when you couldn't before. the most alarming thing, he has and thirst that
can't seem to be quenched. He will not hang out anywhere that he can't get
to water quickly, and he'll drink water out of anything: the toilet,
people's glasses, the sink, the dog's water dish, not to metion his own
which is huge. The other night i filled an old cool whip container with
water and put it down in my room for him so he could hang out with me and
still be close to water, he almost finished half of it in one sitting. As a
result, of course, he urinates in huge amounts at a time. I'm worried, he
seems to young to be knocking on death's door, but this is not normal
behaviour. Please help.