Lots of detail below, but here's the main question: Assuming that a cat has a large hairball and is repeatedly trying to throw it up but failing, and assuming that laxatives have been used but not succeeded, what options are there to help the cat vomit up the hairball?
Way more detail:
Jack is an orange tabby rescue, about two years old. I've had him for about his whole life. Historically he has sometimes (once every couple weeks) made a wheezing sound as though he was going to vomit up a hairball, but then expelled nothing, often swallowing at the end. He has never thrown up a hairball in these two years. I assumed hair was being expelled in feces, so I wasn't worried.
Jack is very overweight. He clocks in at 18 pounds. My other cat, a male, two year-old bombay rescue, has the same diet and exercise but is a healthy weight.
Three and a half days ago Jack started repeatedly (several times an hour) retching loudly, as though he was trying to vomit a hairball and failing. I took him in for a veterinary appointment about 24 hours after this began (earliest I could get). The vet couldn't appreciate evidence of a blockage and I reported that Jack was still eating, drinking, urinating, and defecating, so the vet gave me Amoxicillin for Jack (in case the sound was actually an upper respiratory infection) and laxative paste (in case it was a hairball). I've been using these faithfully since receiving them.
Jack did indeed vomit one hairball up later on Saturday, the first I've ever seen, but it was only a normal size where I was expecting something huge. After this Jack's retching reduced to about once an hour, a third of the frequency it had been previously, but it did not stop. Jack seems to me to be tired or lathargic and to be unhappy. He's less prone to purr and I think he's eating less, but he definitely is still eating. Fat aside, he seems bloated to me. I'm planning to take him back to the vet for an x-ray asap.
I have more than enough money for surgery if he needs it, but I would very much like to avoid putting him through that if at all possible.