Vet doesn’t know what’s wrong but problem isn’t going away.

Hello yall, I’m at a loss. Over the last two weeks my dog has started having these sort of sneezing/reverse sneezing attacks ( https://imgur.com/a/gWpgc0c )

I’ve taken him to the vet twice for it, had x-rays done, and several other tests that came back negative. Long story short, they’re not sure what’s wrong. They have prescribed doxycycline and prednisone and we’ve given to him as prescribed.

The problem isn’t improving and I’m at my wits end. $900 between the two appointments and no answers.

Anybody have any insight into what this could be? I linked an Imgur video above of one of these attacks.

Coughing/sneezing attacks, worse at night. He’s an eight year old terrier/mutt. About 28 pounds and in pretty good health generally.

Thanks in advance.