Hi Again,
Keep an eye on him, and as long as he is eating and drinking, he doesn't sound too very sick at this time. But, if he starts vomiting or has severe diarrhea, I really recommend that you get him to a vet to be checked out. I had a dog when I was living at home with my parents, and she contracted distemper. She started out with diarrhea, and then vomiting and couldn't eat or drink. She got really, really sick and eventually developed encephalitis (inflammation of the brain) and we had to put her down. It was very hard.
I hope that your dog only has a slight virus of some kind, or perhaps, if you do not have Air Conditioning, or keep him outside, he could be suffering somewhat from the heat.
If he does have a temperature, or gets worse, please take him to the vet's, even if you have to go to the emergency vet, especially if he loses his appetite and stops drinking.
With the joint pain and stiffness that you mentioned, it would be a good idea anyway, to get him checked out and he should be getting his shots.
Do you have other animals, dogs or cats? That is always a concern too, as they could contract what he may have.
Here is a good website to visit for more information on dogs and their diseases and problems:
http://www.vetinfo.com/doglame.html#Lameness%20and%20loss%20of%20energy
You can back track on this site, but it does have a great amount of information on problems in dogs with stiffness in their joints, etc.
Hope this helps! Good luck and take care!
Elena Marie
"Open your heart to learn all you can in this life, and to give to others, the joy and satisfaction that they may remember you as a true friend."