I'm not a doctor, and don't pretend to be one. If I were in your situation, I'd trust the doctor's recommendation, and start the Betaseron. Several lesions, optic neuritis -- both point to MS. (I know nothing about an "elevated SED rate", so won't comment!) If it does turn out later that it is not MS, you can always stop the Betaseron.
It is an injectable drug, and there are side effects...but the potential to slow the progression of the disease is quite promising, and as you noted, people are encouraged to start these drugs early, before the disease progresses to the point of disability.
What further testing has he recommended to confirm..or dismiss..an MS diagnosis?
...I am not a doctor, nor health professional, and don't pretend to be one, here.....