Sounds a lot like RSV to me. My son had it in November at 26 months old. He ended up hospitalized overnight in order to get oxygen and IV fluids. It was pretty awful to go through but he bounced back really fast. What I learned from our doctor and the internet is that RSV is an
extremely common and highly infectious illness. A lot of people, including adults, get it and think it is just a common cold. In children under 3 it can be more severe, although even then the need for hospitalization is rare. Is your son still nursing? I think that is what kept my son from getting even more dehydrated than he did. He was not eating or drinking but he was still nursing a lot. After they drew his blood in the hospital they told me he was not as dehydrated as most of the kids they admit there for RSV. I guess somehow from his blood they could tell.
Our timeline was: Monday afternoon DS got sent home from preschool early for being coughing and feverish and miserable. We went to the doc right away that evening because he also had green crud coming out of his eyes. Doc (not our usual one) diagnosed croup (?) and gave us antibiotic eyedrops as well as oral abx (which I saw no point of and did not give to DS). Doc said he would get better in a few days. Tuesday DS still had a fever, a lot of coughing, was miserable. Wednesday and Thursday were more of the same. Around 2 am Friday morning DS woke up coughing and crying and patting his chest and saying, "Hurt...tight." He was also kind of wheezing. This alarmed me a little--if I'd had any clue to really carefully examine his breathing, I would have taken him straight to the ER, but I just didn't know. Friday morning we got a 9 am doctor's appointment and saw our ped. She listened to his breathing, measured his blood oxygen with the E.T.-finger thingie, and sent us immediately over to the hospital for a chest x-ray. She was suspicious of pneumonia at that point and had us admitted and oxygen and IV fluids started immediately. The chest x-ray came back clear of pneumonia and at that point they ran an RSV culture. This consisted of dripping some saline into DS' nose and then sucking out a glob of mucus with an aspirator. From that we got a diagnosis of RSV with a secondary complication of bronchialitis (this is a narrowing of the air passages, similar to what happens during an asthma attack). They told us there's not really a treatment for RSV/broncihalitis other than to keep his blood oxygen levels up (he got oxygen through a nasal cannula, which he didn't like but it is not invasive or painful) and to keep him hydrated. Since he hadn't eaten in several days, they switched him from the saline type fluid to something they called "nutrition." I guess it had glucose and vitamins and stuff, I don't know for certain, but it was more than just a re-hydrating fluid. Several times we had been told to prepare to stay in the hospital through the weekend, but the happy ending to this story is that around 7 am Saturday morning, DS woke up like a firecracker! He wanted down off the bed, he was running around the room dragging his IV pole--I mean he clearly felt like a million bucks. When the doc came in to see him she said, "When they're feisty like that it's time to go home." We were discharged around noon and DS was pretty much totally cured by then. They had us give him nebulizer treatments with an asthma drug for the next few days at home, but it was kind of weird because they also told us there's not strong evidence that asthma treatments even help for bronchialitis. Whatever, he got better anyway.
As for the daycare situation--don't beat yourself (or them) up too much. He might not have even picked it up there. We told our preschool director about the RSV diagnosis but she said no other child had been sick, even tho DS had been there coughing that first day he came down with it. We also got conflicting advice from two different peds at the practice we go to. One said he couldn't go back to school for another week because he was still contagious. The other (our regular doc) said it really didn't matter because he had probably picked it up there anyway. Since he seemed really healthy by that point, and I needed to go back to work, I only kept him home that Monday and Tuesday and then I sent him back. As far as I know no other children at his school came down with it at any point. So I don't even know if he acquired it there in the first place. Could have been from somewhere else altogether. Kids get sick, it happens.
What I learned about looking at the child's breathing is to pull up their shirt and see if their skin is sucking in around their ribs when they exhale. That means they are really struggling to breathe. Also the thing about DS saying "tight." I really wasn't sure what he meant by that. He knew about socks or shoes feeling tight, and was using the word appropriately, but I didn't realize the significance at the time. When I mentioned it to the ped later she was surprised he had been able to describe what he was feeling so well at his age. She said "tight" is exactly how his chest would have been feeling. So he was trying to tell me what he was feeling, I just didn't know what it meant.