Yes, an absolute shocker. I was diagnosed in a much more traditional way, based on symptoms as well as test results.
What you (he) could do is ask for more tests, for example, lumbar puncture or evoked potentials. Something like 80 to 95% of people with MS test positive for Oligoclonal bands in the cerebrospinal fluid. An LP isn’t a pleasant test but it’s more evidence to add to the MRI tests. Have a look at https://www.mstrust.org.uk/a-z/diagnosis The MS Trust has many information pages which are also available as printed booklets, so there might well be more there than just information about diagnosis which will help.
And of course, it maybe that you both need to talk more with the neurologist, to establish exactly how confident s/he is of the diagnosis based on the evidence they’ve seen. Also, whether a disease modifying drug is appropriate for your husband. He may never have had any symptoms, but sometimes prevention is still a good idea. It will perhaps depend on the MRI evidence, ie, where the lesions are and what that might mean in the long term with regards to disease activity.
All the best.