Actually, at first, he WAS hiding, in the bed he had under a shelf, very secluded. He went there when we first brought him home, as a hidey hole, and then started going there a lot, and when he got sick, he refused to leave unless we picked him out. The vet said we caught it early, but that there still was a chance of it getting serious, as he had a very high fever (he had 100.5 when we adopted him, and 103.5 when we brought him in).
The good news is, he's eating now, drinking, and is being social. He slept in my bed with me all last night, and this morning ate some food and drank. So he's at least feeling better. Still gotta continue antibiotics and probably water syringes if he doesn't drink enough, he has yet to use the litterbox since saturday night.