I suppose you could always give the interferon a try to see how he tolerates it. I have been on Betaseron since 2002 with only one new lesion. Sometimes its a crap shoot. Perhaps your doc has other patients on Interferon and feels he sees better results.
Sometimes the disease reacts differently in different people regardless of the treatment course.
You can do two things:
Get a second opinion or
Give an interferon a try

The general rule of thumb is...if one of the treatments does not to show signs of slowing disease progression, then try another drug. Personally I would want to try another drug. I've been stable for quite some time now on Betaseron 1b. Just my opinion.