I can very much understand your dh's reasoning. and yours. I have ms and although it's currently mild the thought of being too ill to work, (or walk the dog), frankly terrifies me. I think you and dh need to have a serious talk about how he;s going to handle his retirement - because this is about much more than a dog.
you say the dog will give him a purpose and opportunities for socialising. There are other things that he could explore that could do that, without you having the joint responsibility of a dog. he could even look at walking other people's dogs (on good days). have either of you been in touch with the MS Society? iirc they have good local support networks. gardening would be a good thing to get into - esp if he could get his hands on an allotment - very sociable I believe. and he coud grow lovely veg for the family and the neighbours. that woud be a good purpose. (and you don't have to do it/commit to it everyday. of course)
If he has only just retired then he almost certainly needs time to adapt - he may be v. scared right now, which is why he wants the dog. but he may well find it's not as bad as it seems.
if he is dead set on a dog then you could explore ways of seeing if you can be persuaded. I know it will be hard to convince you, as the bottom line is you don;t like them, but not all dogs smell, not all dogs chew (in fact the vast majority of adults don't chew), there are some breeds that don't shed hair, and there are some that don't need much walking (greyhouds, for example, need abt 20 mins a day). many are well trained and don't jump up. you could also look at getting an elderly rescue dog - ie one with a life expectancy of just a year or two. often difficult to rehome as most people don't want that.
I don't think either of you abu. I just think you need to talk.
very good luck and best wishes to your dh.