URI symptoms with no improvement, need ideas on how to proceed

Sorry for the super long post, but I’m hoping someone here will have better ideas as to what can we do next. Our sweet 3 year old boy started coming down with what looks like an upper respiratory infection a couple of days back.

On Jan 1st we took him to an emergency vet because he refused to eat day prior, drooled excessively, was lethargic, had discharge from his nose and licked his nose raw trying to clean it up. The emergency vet gave him anti-nausea shot, painkiller and anti-inflammatory shot and recommended that we take him to our regular vet once they are open. After the visit, he was clearly feeling better, he ate his food normally and got somewhat more energetic. He still wasn’t himself, but he was interested in what was happening around the house, following us around like normally and even meowed for food. I do think it was mostly the effect of the painkillers, as he got progressively more lethargic as time passed.

We took him to our usual vet the next day. He examined him and said he hears some murmurs and suspects an upper respiratory infection. He prescribed a wide spectrum antibiotic and an anti-inflammatory liquid medication. We gave it to our cat, bur he wasn’t really improving. He went back to refusing food, only accepting the liquid tube snack and not in large quantities. He clearly was hungry and tried to eat his normal wet food, but kept gagging on it and pawing at his face in pain. It’s worth mentioning that he usually eats around 400g of wet food a day (he’s a maine coon, so he’s pretty big) and at this point he could manage a few bites of wet food and 3-4 tubes of the liquid snack.

So we took him back to the vet on Saturday. This time he got an x-ray taken, the vet said they don’t see anything in his lungs, but it looks like it might be bronchitis. Additionally, his palate was super inflamed, but they couldn’t tell of it was from the liquid medicine or the URI. His antibiotic got switched to doxycycline, they stopped the liquid anti-inflammatory drug and gave us to give him buprenorphine for the pain. Told us to go back for a check up on 7th Jan. At first there was some improvement, he went back to eating with the pain meds, but other than that he was still pretty low-energy, lots of sleeping (which I know could also be exacerbated by the buprenorphine).

However yesterday evening and today morning he went back to acting like he did on Jan 1st - constantly sleeping, drooling, congested nose, he wants to eat, but can’t, he’s even refusing the tube snacks he ate before.

I’m stressed to not know what is wrong with him, because I feel like we have no way to even start forming a plan to help him and he’s clearly in a lot of pain. Additionally, my anxiety is skyrocketing, making me convinced it must be something big like felv, but I’m not sure if it’s just anxiety or it’s a real possibility. I’m thinking of asking the vet tomorrow for a felv blood test?

I’m running out of ideas on how to help him. Are there any tests or other diagnostic actions I could ask of the vet tomorrow that could be helpful? Additionally, if he refuses food today, should we try force-feeding him with a syringe, so his liver doesn’t end up going bad? I can’t get him to the usual vet today, because it’s a bank holiday and I fear the emergency vet will only stress him out and not give us any real answers.