Whew, AC. I am glad that it is an allergy - but it is still very serious - I have heard of fatalities from allergies to enhancing agents used in both MRI and (more commonly) CT scans (they use an iodine-based contrast for CT), scary stuff, so it is really good that you have discovered this. These allergies seem to get worse with each exposure, which is a shame as MRI scanning with contrast is so useful with ms, but it is not essential to treatment. How is he feeling now?
I don't know all the science (you will know much more), but isn't how the body reacts to an allergen similar to how it acts during an ms flare, but with ms it is attacking the CNS as the 'foreign body', not the allergen? Also, there is thought to be some kind of a blood vessel inflammatory relationship to the initial formation of ms lesions, so do you think this vasculitis/allergy may be in some way related to his ms?
Anyway, good thoughts and prayers to you and your husband - as Lew said, it is especially awful to be hit with another illness on top of ms, but from reading your posts, you seem to be very strong together (you and your husband) so in that way you are blessed and that strength will get you through this.