It’s his choice to take it (but he doesn’t have MS, so he’s hardly proof of a “cure”). Equally, it’s your choice not to.
“Might do you good (if you persevere for a year or two)” is simply not good enough. Life with MS is hard enough, without adopting foul regimes with no proven efficacy.
If you want to engage in “tough medicine”, force yourself out for a walk every day, or some other kind of exercise. It costs nothing, won’t harm you, and my neuro reckons it’s better than anything he can prescribe. I’m not necessarily including DMDs in that, as I’m not sure exercise actually changes the disease course, but it certainly helps strengthen and maintain whatever physical capabilities you still have. Which drinking some faddy mushroom concoction won’t.
The argument about not taking a cure if it tasted awful is completely unfair, because what’s he’s offering isn’t a cure, or even an exciting candidate. Of course you’d put up with a little unpleasantness (or even a lot!) if there was good, strong scientific evidence it was going to work. But on some very slim and unsubstantiated chance it might make you feel a tiny bit better if you persevere for a year? No!
You’re not “old school”, who still promised to obey, are you? Even if you are, I think the time has come to decide what goes into your own body, and whether YOU have faith in it - not whether he does. It’s not his illness, and not his choice what you do about it.
I don’t want to stir up further trouble between you, but what he’s doing is bullying. Rejecting quack medicine doesn’t make you pathetic or ungrateful. It all seems to boil down to him not accepting your diagnosis, and convincing himself that if you don’t get better, it’s because you’re too pigheaded to try. This is delusional thinking. I understand why the delusion is attractive to him, but it’s no help with your real situation. You’re basically being told it will be your own fault if you don’t get better - essentially that having MS is a choice you can avoid if you try hard enough. Yes, that would be very nice in his rose-tinted world, but nobody ever fixed MS by trying hard. What he’s refusing to accept is that nobody ever fixed MS at all.