Help. Severe sinus infection

Just rescued my pup from a local county shelter yesterday, and they sent him home with some doxycycline stating they think he might have an upper respiratory infection. Turns out he has an INTENSE upper respiratory infection, with a bad case of post nasal drip, nasal and eye discharge, and coughing to the point of gagging. I’ve already done a couple of 15 rounds of steamy bathroom for him to try to loosen the mucus but it isn’t cutting it. He can’t sleep, I can’t sleep, and I neither of us will make it 14 days of this. Any suggestions? Local vet opens at 7am. Is there a difference between them and an urgent care? I can’t wait the time it would take at an emergency center because his situation isn’t life threatening and anything else would go in before us, understandably so. Thanks in advance!

A couple of 15 minute rounds*

The majority of cases of acute sinusitis in dogs are caused by viruses. Typically, canine flu, parainfluenza, canine distemper, and canine adenovirus types 1 and 2 are to fault.