Hi there, think i would try for a second opinion vet- wise if i were you, "probably a respiratory illness" sounds like a i'll give it a name, but i dont know what it is diagnosis, would it be possible to try either vets Rodentopia or sport_billy recommended?
If you can get a probiotic for him to help replenish his 'good' gut bacteria this should help him improve too, his digestive system must be kept going with food in & poops out, or the gut can go into stasis (stop working) which can cause major problems.
(might be worth a probiotics that are added to water &syringe feed him some of that, you can monitor his fluid intake better then too-use a small syringe to the side of the mouth, so it doesn't go down the wrong way) or wash some freshly picked grass&sprinkle probiotic on it, he might be able to manage strands of grass or hay more easily.

i would definitely try syringe feeding him-theres quite a knack to it(personally i'm rubbish at it)even mushed up pellets in hot water,then cooled (if you have no recovery food available),but little & often sounds like it may be the way to go with your little man.
if you can hear crackles it sounds like he could have some excess fluid going on in there somewhere, you will need to get this sorted,probably with a diuretic. if a piggy is given the option to breathe or eat... then breathing wins every time... no matter how hungry poor piggy is.
I have used olbus oil dripped on to a cloth near the cage/hutch but out of reach from my piggies before &it does seem to ease their breathing.
Checking the teeth would be good, may be an issue there like you said.

Good luck with Cola, wishing him a speedy recovery,plenty of healing vibes & a hug for you from the girls & I. xx