Hey guys, just a couple questions

First time rat owner here. This is going to be a bit of a novel, so sorry in advance. I adopted a rat who I instantly fell in love with at Petco last summer, was on the "Adopt Me" table and was going to be put down that weekend if we didn't grab him, so I did.

I use Careresh pet bedding, Kaytee fortifood diet (not like that matters, he isn't eating any of that that I can tell), I clean his cage every four days, disinfect everything every other cage clean with a bleach cleaner, and very very thoroughly scrub. Fresh fruit every morning and evening, especially now since it seems to be the only thing he wants to eat. He has a barred cage with plenty of ventilation.

Getting to the point, he got sick last December, wasn't breathing very well and was very very lethargic, stopped drinking. I put him on baytril tablets, which I split into quarters and gave to him mixed in a random sweet (apple sauce is his favorite). After I was out of the medicine, it lasted about a week, he was fine for a week, but then went into a relapse in early January. Got two weeks of medicine, and he was fine for another two weeks. Then in February he started to feel bad again, this time worse outta all, he was breathing so hard he had to put his all of his muscles into it, and squeaking as if it hurt each breath. took him to a different vet, he was diagnosed with strep throat, so he was on amoxy-drops. That worked for a week but then he got sick a week later, took him back to that vet, got diagnosed with lobar pneumonia, and put on three months of doxycycline--which he responded to for a week, but has just recently relapsed again. Took him to a different vet, finally one that loves rats, and she put him on .25 cc of doxycycline and .06cc of baytril together. I also am giving him one drop of Beano a day to deal with the excess gas he is getting from his "breathing" bouts, where he seems to be gasping for breath (this doesn't happen a whole lot anymore, just when he gets too stressed). Also a humidifier stabalizing at about 65% humidity in his cage.

He seems to be in limbo at the moment, he doesn't really want to do anything but sit inside his igloo and stare out at me, but he isn't making a lot of noises while he breaths and is...kind of eating, but very sparingly. He likes his fresh fruit and apple sauce. which is both keeping him sustained and hydrated, to a point. I take him out and he wanders around a little bit before nuzzling into my neck and making this lil' grinding noise with his teef. Breaks my heart to see him like this, and I am considering putting him down because he apparently has a very nasty gene pool, but I don't want to if I don't have to--he seems to be functioning.

So really my question is, at what point of functionality should I consider euthanasia? I really don't want to if I don't have to, the thought nearly brings me to tears, but I know I need to put his feelings into priority.