Cindy --
I have a slightly different opinion from many here. If you have ALS there is no reason to be in a hurry to be diagnosed. Rilutek is the only FDA approved treatment and in tests it extended life (but not functionality) by only a few months. Additionally, it's expensive and very hard on your liver. I don't take it and neither do lots of other PALS (not that taking it is wrong--it's just far, far from a wonder drug and many believe it hurts more than it helps).
IMO, the only reason to be in a hurry for a diagnosis is if there is any chance you have a treatable illness. And that's where your doctor comes in. If I were you, I'd press that sad-sack young doc to tell you exactly what he thinks you have, complete with probabilities. Use that information to decide your next move. If he thinks MS is still anything more than a very distant possibility, get yourself evaluated at an MS center--there are a number of effective MS treatments out there. Ditto Lupus and MG. Once the treatable things have been ruled out then you have a decision to make. Do you want to know for sure if it's ALS? Only you can decide. But, practically speaking, there's no advantage to a diagnosis unless you need services you can't qualify for without one.
And, now that I'm thinking about it, this entire analysis assumes you have insurance that will pay for all these evaluations.
Good luck.