my cat pants like a dog through his mouth, is this normal?

Maybe he’s stuffed up,or worst case scenario,he has a heart condition.

Cats only pant like that when there Is something wrong with their lungs or when theyre having an anxiety attack. Since he had previous respiratory problems my best bet would be that there is something still wrong with him. cats shouldnt pant like dogs..ever. So go gt him checked

It sounds to me as if he may be either extremely dehydrated, or he’s having difficulty breathing. You should take him to a vet.

Typically, cats only do this when they are stressed or ill.
However, since you have obviously done the right thing and taken this to your vet, I would be inclined to tell you not to worry.
As long as you have an idea of where the panting is coming from (eg: exercise) don’t worry. If, however, you see your cat laying/hunkered on his stomach, with his elbows pointing out, neck extended and panting – and you can’t find a reason for it, call your vet immediately.

I have had 3 cats who have panted as you are describing. Two were “fetchers” who would chase a ball down a hall and bring it back (like a dog) over and over again, until they would flop down next to me and pant, at which time I would end the play. The other was a “feathers on a stick” junkie who would do flips in the air over and and over again, chasing the feathers. After several minutes of flipping, hopping and jumping, he would start panting and I would put the toy away.

Enjoy your cat!