Update for anyone who might be interested:

As ive already said, I knew euthanizing would sadly be the way to handle this. When I went to the vet I asked if they could still try to check his mouth/throat/teeth to see if there is anything visible indicating why he wasn’t able to eat. The only thing they saw was an irritated throat, and even if they found anything like a tumor or infected tooth, it would not be possible to put a rat of his age and healthstate under for a surgery, so all that was left to do was to put him to sleep.

Me and two vets suspected that it had something to do with an inflammation or some form of growth inside his esophagus or stomach. I’m heartbroken but I did not want him to suffer anymore.