Parvo is no respecter of age. It can happen to any age of dog, just one who hasn't had his innoculations....
But yes, you would notice diarrhea (usually very foul-smelling, and probably with blood)and most likely vomiting as well.
You could try mixing some boiled (and cooled) water with a teaspoon of honey or maple syrup, and a pinch or two of salt, if you can't afford something like Pedialyte. That will help slightly if there is any dehydration.
But it is my opinion he should really be checked out by a vet.
There is absolutely no way to tell what may be wrong with your dog. It could be any number of things. If it were Parvo, you would probably have some serious issues going on like diarrhea and probably blood, for one.
I am not trying to be judgmental but if you can't afford basic Vet bills, maybe it's not good to have a pet. Try feeding some poached chicken and rice and see if that helps. Make sure the dog is drinking water. Sorry, probably needs to be checked out.