Have you sat down and spoken to your Dr and done some research in to your MS and what the possible effects on you could be in the future?
If not, then maybe you and your fella can do it together. You'll learn more about it and both your questions and fears can be answered together. Also it might lead on to conversations about what you'll should such and such happen.
Otherwise perhaps just sit down and talk to him about your fears. I'm sure he'd appreciate your honnesty.