The first thing that comes to mind is Coxsackie (hand, foot and mouth). Symptoms are blisters and sores in the mouth and red spots on the hands and feet, however, i've had it and my kids have had it and never had the spots on the hands and feet- just the mouth sores. It's extremely painful to eat or drink, which would be why he's avoiding it. There is no medicine for it, it's a virus that needs to take it's course, but there is numbing medicine they can give him topically for the sores on his mouth.
If he's only had one wet diaper in 24 hours you need to take him to the hospital. He needs an IV. The rule is one wet diaper every 12 hours (we learned that last year when my 13 month old had H1N1). Don't hesitate, take him to the hospital now for the IV. Being dehydrated is probably making him feel worse, which is making him not want to drink even more. Take him now!