4 month old GSD with a temperature of 105?

Our puppy woke up this morning with a seriously high temperature. We rushed him to the vet and he has been on fluids since 8am. They ran a blood test and it came back clear, slightly elevated white blood cell count. His lungs and eyes are perfectly clear, heart rate and blood pressure were normal. He is lethargic but other than that and his temperature he is seemingly 100% fine. Obviously something is very wrong though, if he isn't improving at his normal vets by 4 we are bringing him to the university of Pennsylvania.

I just wanted to see if anyone has ANY ideas- he is fully vaccinated and has had no health problems to date. Any ideas would be welcomed- thank you.