This is a difficult one, because he may have had genuine intentions, and be seriously wondering how you’re going to manage (which you may be wondering yourself).
Nevertheless, he’s not allowed to simply suggest you shouldn’t be working there. At the very least, he has to look at ways to support you to stay in work, even if that means changes to your hours or duties, or perhaps some kind of aid or device to assist you.
It’s premature while you’re still off sick anyway, as it’s not yet clear what the extent of your recovery will be.
Unfortunately, depending on the type of business, it can sometimes happen that a person with MS just isn’t able to carry on with it, because the employer doesn’t have any other duties they could switch them to, or has already exhausted all the ways of trying to help, but still nothing was able to overcome the problems.
BUT, you’re a long way from that yet. You’re not even back off sick, but when you are, there needs to be a long hard look at your duties, whether you can still cope with them, and what your employer can do to help.
On no account resign, because they claim it would be better for you. If it gets to the point it’s clear you can’t continue, make sure they terminate your employment on health grounds, as it may greatly affect what severance terms you leave with, and also what benefits you’re eligible to claim. If you resign, it will be viewed as any ordinary resignation unconnected with health - i.e. entitled to nothing, and may not get benefits either, due to possibly being viewed as “voluntarily” unemployed.