Well, after much debate over whether to take him to the vet or go to work, I opted for the wait and see with Ares and went to work. I'm home now, and he seems about the same.
His attitude and appetite are good. He seems to have a stomach ache, and is quite sore in his hind legs and had two peeing accidents today. So I am somewhat clueless as to where the problem originates from, but I'll keep an eye on him for now and see if he needs to go to the vet tomorrow.