Someone else can probably chime in on this as well cause my experience isn't as vast as some of the awesome people here....
What kind of bedding do you have him on? I know I caught my biggest snake 'gagging' after shedding because while bumping about she gets some coco fiber in her mouth. She hates the taste.
I don't know if this could have been the same with yours.
For the refusing food...they hate the meds. It will sometimes take a bit after being on oral medications for them to go back on feed. My last snake treated for bacterial infection took almost 3 weeks to go back on feed. A week after meds to stop being headshy. I don't expect them to eat at all while on meds but do still offer something just in case. Right now tho he probably expects the check and icky taste so yeah, he will avoid you.
As far as symptoms...
The bubbling your producing is most likely stress related right now. The more pressure you exert by trying to hold and check the snake over, the more pressure on the stomach. And they will sometimes drink a large amount of water while on meds.
I would avoid as much handling and mouth checking as possible as this can actually cause him to inhale fluid and get pneumonia. Try to only medicate for now and monitor.
The symptoms you want to watch for are:
Extreme lethergy and weakness
Sudden weightloss
Open mouth breathing or headbobbing
A film or milky crust around the nose and mouth almost like stuck skin
Basking in strange places with the front end of the snake raised (usually accompanied by the headbob)
Watery stools
Hopefully someone can chime in with more info, but these are my experiences~