He doesn't want to commit until you can stop your SI? That is a reasonable request, but what about that whole "in sickness and in health" thing? You're not perfect, and even when you do finally recover, you might have relapses down the road (hopefully not).
I'm sorry to sound pessimistic, but that sent a big red flag up when I read that. I hope you guys do get married eventually.

Have you asked him why he wants to wait because of your SI? What difference does it make between now and with a ring?