Purple comb just means he has poor circulation, it is not a diagnosis, it is a symptom some severe disease process. Yellow fluid from his mouth could be from his crop (food mixed with water/saliva) or from his lungs/air sacs. That with lying there- the bird is likely dying. If the other birds are totally normal, he may be the first to succumb, or he may have something not contagious.
If he had lost noticeable weight a few days ago, he likely has not been meeting his calorie needs for a week or longer- doesn't fit as well with an acute respiratory problem like avian flu (incidentally regular old avian flu is around, but the 'bad' asian avian flu that can jump to people is not here yet). If you have many ill birds with severe symptoms and mortality- it would be worth it to send one to the state lab, most states if you get them the bird will do a necropsy for free and tell you what pathogens you have in your flock.
If everyone else seems fine and stays that way for the next two weeks, they probably are fine.
With a single very sick bird and a bunch of healthy ones, it is much more likely he has/had a non contagious problem like a wound you did not know about, ate some toxin/poison, or even had some genetic problem that caused something internal to fail at an early age.
Antibiotics won't help him if it is viral or ate something toxic, but they might if he has a bacterial infection or hidden wound.
Hope he is not as sick as he sounds, Jess