Hi, Seacoast. I had to process your post for awhile. I had a friend die of ALS too, so it's hard for me to even contemplate this disease.
There are a couple of things that bother me about his doctoring. One, he doesn't have a diagnosis yet. He's already been given Mestinon and Pred. Mestinon, as others like Desert Flower have said, is a tricky drug. It's often recommended by MG experts to take it at a lower dose at first. The dose he was given for Pred is awfully high. And if he doesn't have MG, why try it? No, I'm not saying to go against doctor's orders but you have to question why doctors do things sometimes. Pred is a very hard drug on the body and it's also very hard to get off of.
Also, while on Pred and Mestinon, the EMG's and antibody tests can and do go negative when they are in fact positive. If you don't have copies of the EMG test results, please get them. They may tell you more than the doctor has!
Was this neuro an MG expert? The SFEMG is more specific for MG, however, it is not diagnostic of MG. The results have to be taken in combination with the clinical exam and other testing. Has the doctor actually said "You have MG?"
Has he seen a neuro-ophthalmologist, which is not the same as a regular eye doctor. They can assess muscle weakness of the ocular muscles and see if he has double vision. I didn't realize that I did indeed have double vision until I saw a neuro-ophthalmologist.
Have you taken any photos of his face? Can you tell if any of his facial muscles droop?
Sometimes with MG, you have to utilize a few different experts in the diagnostic process. It also helps to see a pulmonologist. They can assess if breathing is fatigable by doing the MIP (maximum inspiratory pressure) and MEP (maximum expiratory pressure) tests in addition to the "regular" pulmonary function tests. They might be able to determine if any reduction in breathing is "fixed" or "variable" and may give them more clues.
Mestinon has "normal" side effects (see PDF below). So many patients do not read drug information before they take a drug. Sometimes you can have side effects for a couple of days and then you won't. What dose did he start on?
I hope he doesn't have ALS. One thing you might want to consider, in addition to MG, is a B12 deficiency. It's very common in people over the age of 50.
Please keep asking lots of questions of doctors. Or here. When facing a serious disease, it's important to keep an eye on the details. For example, if he has MG, certain drugs can make him worse. Other things like stress, infection, surgery, hot weather, etc. can too.
One other thing they often do with MG is take a chest CT to look at the thymic gland (thymus). Some people with MG can have a tumor of it. If they "insist" on Iodine contrast, be aware that it can make MG worse too.
I hope you get answers soon. Take care.
Annie