Completely agree with Jane - you are not automatically allowed time off for a scheduled hospital appointment (i.e. not an emergency admission).
However, the thought occurs to me that if you are in a relapse, you would probably have had no trouble at all getting signed off sick - if you’d wanted to. I’m not saying that’s the way to go - if you can work, and want to.
But the irony is that if you had, you’d have had no work commitments, and could have gone to the hospital whenever they said.
So, whether he means to or not, the message your boss is sending is: “Next time, it’s better to go sick.” It’s tricky, because you don’t want to sound as if you’re trying to blackmail him - it won’t be good for future relations.
But maybe you could try pointing out that an MS relapse is usually ample grounds for being off sick altogether. You’ve helped him out by choosing not to take that route, even though you could have. But now there are obstacles around getting outpatients’ treatment. So clearly, from your point of view, going sick would have been better. It’s a tricky one, because a lot of bosses take the view that you’re either well enough to be at work, or you’re not, and IF you turn up as usual, it’s assumed you have no more compelling claim for time off than anybody else - because you’ve reported as fit!
Maybe you learn from this experience, and next time you have a relapse (I hope there won’t be a next time for ages, of course), you just say: “S*d 'em all!”, and go off sick.
Not the best attitude, but if it does away with this hassle about whether or not you’re “allowed” to go to the hospital for steroids, you have to do what’s best for you. They are silly people! They haven’t encouraged you to try to keep working where possible.