I am so glad you were able to have your rat on oxygen. That really can make all the difference.
I still think an Xray would be helpful, myself. There could be more going on than you can possibly know without seeing an Xray. I believe I already mentioned I just recently had a rat who was having difficulty breathing and had Xrays taken. The Xrays revealed she had a tumor inside her chest that was pushing her heart to one side.
Have any of the doctors recommended nebulizing? In addition to warm, steamy bathrooms, nebulization can be a huge help.
Here are a few nebulizer treatment links:
http://www.rmca.org/Articles/albuterol.htm
http://archietherat.com/nebulization.html
This is where I bought my nebulizer:
http://www.lambertvetsupply.com/Schuco-S5000-Nebulizer_p_2430.html
In case it's helpful to know about, there are lots of variables with nebulization....many different ways to do it. What's worked best for my rats is to have them in the bathroom on my shoulder or in my arms while I steam it up with a hot shower for about 10 minutes. After the bathroom mirror is fogged up, I turn off the hot shower and stay in the bathroom with my rat(s) for the rest of the 10 minute period. (I use my cell phone as a timer and place it inside a drawer so it doesn't get steamed up, too!) The steamy bathroom helps open up their air passages so they can take in the nebulization treatment even better.
After the steamy bathroom treatment, I allow the rat to be in their cage while I get a small carrier pre-misted with my nebulizer. To do this, I place a large towel over a very small carrier. (I have one that's a little bigger than a loaf of bread.) I first mist the inside of the carrier without my rat(s) in it for 5-8 minutes. (I have a Schuco S5000 nebulizer.) Then I place my rat(s) inside the carrier (that's been pre-misted) for 5-8 minutes. While they're inside I feed them healthy treats so they are less stressed out. (Examples of the healthy treats I feed are frozen peas that have been heated up, giving them one pea at a time; dried edamame or soy or garbanzo beans---again one at a time.)
This is the mixture of medications I've found successful for nebulizing. I place the following into the nebulizer cup and have this all set to go before I begin the steamy shower treatment:
Gentamicin 0.25 ml
Albuterol 0.25 ml
Saline 3.5 mls
I have definitely seen significant improvement when doing the steamy bathroom followed by the nebulization. For my rats, sometimes just the steamy bathroom helps enough. But if the symptoms are severe, I add the nebulization. I only do these treatments when my rats' veterinarian recommends it. Usually my vet will say (if my rat is very sick) to go ahead and try it and see if it helps. Not every rat will respond....so it's usually worth a try. Obviously, if the rat seems worse at anytime during the treatment, I immediately stop.
I agree with what someone previously wrote----it does sound like your emergency vet is much better than your regular vet. Can you ask the emergency vet if they know of any great rat veterinarians in your area? Where I live, a lot of vets work part-time at emergency clinics and also work in day practices and they're usually happy to give advice on the best places to take your rats for veterinary care.
I also agree with the person who wrote that veterinary care is not inexpensive and, unfortunately, rats can have significant medical problems.
I'm sending positive, healing wishes for your beloved rattie. You are doing great things for him.
Almost forgot I have a blog post on mixing medications so they're appetizing:
Recipes for Success: Making Pet Rat Medications Appetizing