Hi, Doris, Melinda, Pat, Cliff and Ellie - Thank you so much for your kindness and suggestions to help out my Harry. I decided against going to an internal medicine vet because I have already consulted with four different vets, along with our much-trusted vet of many years (who in the past referred us to a specialist when Harry had cancer), and the only other suggestions than what we were already doing involved doing a CT scan or a nasal probe. The only thing either of those tests would reveal would be something that wouldn't be treatable for a 15-year old in a very weakened condition.
So I've made the devastating decision to say goodbye and have him put down tomorrow. He's been struggling with this for about five weeks and has gotten gradually weaker and weaker, and now it's clear for the first time that he's really in distress, so it's time.
I'll be back on the forum soon (hopefully very soon) to get advice on finding a breeder or rescue organization in my area (NYC) to find Harry's successor.
Thanks again for your kindness,