What is wrong with my dog?

About 5 days ago our dog slowly starting having the following symptoms: lethargy, neck/back stiffness, and a small patch of hair loss on neck. He is a (approx) 2 year old male lab/pit mix with a thick black coat. In addition to the small patch of hair loss in his neck we had also noticed some thinning of the hair on his belly, and yesterday we noticed a small sore in the center of the bald patch of his neck that was definitely not there just a day ago. He has a normal appetite/thirst, no stool/bladder problems, no fever. Ears and throat/teeth look fine. We originally thought he was having an allergic reaction to maybe a wasp/bee sting, or snake/spider bite. Only other guess I have left (after various internet searches) is maybe a type of mange? However, he is not itching at all. I am at a loss and don't want to spend a fortune at the Vet so they can tell me they don't know what it is! Yesterday I put a dose of revolution on his coat to try and rule out the mange. Please help. Thank you.