I meant to ask in my first post to you if your vet had performed bloodwork before prescribing Rimadyl to have a baseline liver value to work with before beginning the medication? Rimadyl, while an excellent medication for relieving the pain of arthritis in canines, can cause liver damage over long-term use. Even though dogs have been domesticated for hundreds and hundreds of years, they still retain enough of their wild instinct that they will hide not feeling well, often until it is too late to do anything to help them. Unless we know them as well as we know ourselves and can detect even the slightest bit of difference in their demeanor, we just don't know that there is anything wrong until they are so sick they just can't hide it any more. I can't help but think that perhaps the Rimadyl is the culprit, I just hope that whatever it has caused is something that can be gotten under control by stopping the medication or changing the dosage. Again, please let us know what the vet says. I am not trying to alarm you, I just need to make you aware of what his problem could possibly be.
Ghilly
I would most definitely consider this to be an emergency situation and would not, given his advanced age for a large breed dog, even wait until morning to go to a vet. I would pack him into the car and take him to a 24 hour emergency vet. From your description of the situation he seems to be going downhill pretty rapidly so at this point every minute is important. If you absolutely cannot get him to a veterinary ER tonight, be waiting at your own vet's door in the morning when he opens and ask to be seen as a walk-in before he starts to see his other patients. Please post back and let us know how things go and what the vet says.
Ghilly
This really sounds like he should be seen by your Vet and ASAP.
Anything I would say would only be a guess and he seems to have a number of symptoms that could indicate something serious.
Hope all turns out and you do get him into the Vet and please update us.
Good luck