First off, wanted to tell you that the proper name for this disease is stiff person syndrome, not stiff man disease. I am female as are many ppl who have this disease. It was once called stiff man but it was changed bc of this.
If your doc is calling it stiff man disease, that suggests to me he may not be very knowledgeable of it. I was diagnosed by a spinal tap. If I were you, I would kindly ask your doctor if he has ever treated this disease this before bc it is very rare, very serious and you need an experienced doctor. I had the blood test and it came back a weak positive, not to mention the symptoms I'd been having. But I'd never heard of this disease before. I also got a second opinion but it wasn't for lack of not believing the first doctor who diagnosed me with it.
Before I went on a med like the one you mentioned, I would want to be 100% sure I had it and the best way to do this is to have a spinal tap. But SPS doesn't generally start with full body muscle symptoms, unless you've had it for years and I would think it would have been diagnosed before it reached that level of severity. . There are a lot of diseases that that could be but then again if you have the GAD65 antibodies, that is usually an indicator of SPS.
I'm not sure what you mean when you say you tried IVIG and it "didn't work". IVIG is used mainly to slow the progression down and it is highly unlikely that you would (or a doctor) be able to tell if it is working or not for a very long time.
There are also several diseases that might show up in a spinal tap that may not in labs. But if this doctor is experienced treating this, then you need to get busy treating it! It will progress and believe me, you do NOT want it to. I know from experience that it is very painful and takes so much from your life that you want to find a treatment that works and asap. I'm not your doctor or a doctor so if he seems to know what he is doing then do what he thinks you should. I wouldn't stick with him if he has never treated it before. The doctor that found mine had to refer me to a doctor that had more experience with it. He told me that and I know he was doing what was best for me.
Good luck. I can't tell how old this post is so if you've already resolved all of this, sorry and the best to you.