Kennel cough diagnosis, is this a normal progression?

I've looked at older posts but was hoping to get some guidance.

Our 10 year old Lab mix started some mild coughing about a week ago and after a day or so developed a kind of retching cough. On Wednesday he spit up some clear/foamy sputum(?) so we called the vet and took him in.

The vet did not do any x-rays or blood work but confirmed kennel cough but said his lungs sounded clear and he had no temperature so she prescribed a cough suppressant and we were told to keep him in for a week or so until the cough stopped. The cough medicine seemed to really help and Wed-Fri night he was coughing much less. His appetite was normal and he was in great spirits.

This morning (Saturday) he has been coughing quite a bit. Much more than the past few days and again is bringing up some phlegm. He did not eat and is pretty miserable. We called the vet and were told over the phone (by a tech not the vet) that this was to be expected, it typically gets worse before it gets better and him bringing up the mucus was actually good. She said try to get him in a steamy shower room which we did and if he didn't improve in a day or so to call again. It's obviously breaking our hearts to see him like this and I'm tempted to call again to insist he get further testing but don't want to be an alarmist.

Can anyone offer any guidance on this? Edit: his breathing is definitely more labored today. Not like he's struggling but definitely different.