What I can say for your situation is you should get some research in, if you haven't yet, on all possible symptoms of MS, then ask him which he has if any presently. Then you should research whichever injection he is taking and how it effects him. I take Avonex, for example, and the day after my weekly injection I feel awful and refuse to go anywhere. The other two interferon shots are the same composition as Avonex, but they are in smaller doses over many days of the week, so the effects are smaller. I am not sure what the last injection is like, but I know it doesn't give flu like symptoms.
Once you know how he'd feel after injections, you'll know what he can do daily and his mood during healthy periods (and you'll know what he's going through after the research for when he gets new symptoms).
I wish for the best because I've often felt worried about surprising a partner about having something like this. I was thinking if I was in a relationship I'd show them sooner than later what my shot did, so I truly can't be of much help, but I answered it how I'd want someone to react. Being knowledgeable will let you understand his limits better and you'll be able to talk about it easier. I know I couldn't be of help, but knowing is the key issue to this problem as simple as it sounds.
Good Luck