Dedra, it sounds possible if they're relating it to MS but that's not the only condition that could cause it. (I know that Pars Planitis is another one.)

Had he ever experienced any eye pain, color desaturation or blurry vision? Mine was really a textbook case but I'm sure you've read about others who've experienced ON more "silently". Personally, if the nerve is implicated (for whatever reason), I'd try to get a referral to a neuro-ophthalmologist.