Hi and welcome, i'm sorry for the difficult situation you and your partner are dealing with, your partners medical team not being very helpful and transparant which is the exact opposite of what you need in your treating medical team!
To be honest it's actually not common for brain cancer to be confused with MS, the closest type of MS these dr's might be talking about would be one of the rarest types of MS called Tumefactive multiple sclerosis, very rare form of multiple sclerosis (MS) but causes similar symptoms to those of a brain tumor
and brain MRI's show tumor-like lesions that are larger than two centimeters.
As to what type of brain cancer they might be refering to is not something i know anything about sorry. I strongly suggest you make a point of getting a second opinion with an MS specialising neurologist to help you and your partner make more informed decisions on what would be the best treatment for the situation he is dealing with.
Hope that helps......JJ