I am sorry to hear you are so worried about Mr. Kitty. I am a veterinarian and will do my best to help.
He certainly sounds like he is in distress, and painful. Could he have been injured? Is there any rat poison, or antifreeze, where he would be able to get it?
If he will take water, that is a good sign. Keeping him hydrated until you can get him to a veterinarian is a good idea.
If you check his gums, are they pink in color, or pale? Pale would indicate shock, internal bleeding, or dehydration.
If he can't walk, he could have some kind of injury or problem with his spine, or he could have a blood clot in his hind legs. If he is panting, he could have a fever, or be painful. Howling could be pain, or mental confusion from a blood clot, or a drug or poison.
It is hard to know for sure what is wrong, without being able to examine him. I would keep him confined and keep offering food
and water, and get him to a veterinarian as soon as you can. Being able to look at his gums, take his temperature (normal rectal temp is between 101 and 102), feel his pulses, listen to his heart, feel his limbs and internal organs.
There really are no good over the counter pain drugs that are safe in cats; Tylenol is poisonous to cats for example, so I would not try to give him anything until he can be checked out.
Sorry for his and your distress. All you can do is keep him confined and quiet and safe until you can get to your vet.
Let me know how he is doing.