I really don't think this is a black and white situation, and I don't think you deserve the pile-on that you're getting OP.
MS is an awful condition, but some people only have mild symptoms for many years. So while I think we're all imagining someone who has a terrible disability, that may not actually be the case. No doubt flying is difficult and uncomfortable, especially if it's got fibro at the moment too. All I'm saying is that there's not enough information in the OP to decide whether DH flying business class really is essential or whether he could get a better seat in economy and be OK. If he's mobile and can move around, he might still be able to sit on the exit row - as I say, there's not enough info about how severe his MS is to judge.
But let's assume that it's fair enough that DH needs business class - and that's not an unreasonable assumption. I can definitely see why that might be the case. I think the issue is that he doesn't see why OP and the DC should get to fly business class too. So DH gets the enjoyment of the business class lounge - which OP and DC won't be able to access. He gets the comfy seat, the nicer food and all the perks. In the meantime, OP gets to watch the DC on her own in the airport, manage them by herself the entire plan journey, have the substandard seat/food/perks etc.
I would expect DH to acknowledge that although his medical condition requires him to take the better seat, the OP really is taking on a lot more of the burden and it's a little unfair. An honest conversation about cost and affordability is needed, but for me, DH's attitude would make a big difference. An entitled attitude with an assumption that no one else should be able to fly business class and that OP should just suck up everything would make me far less amenable to flying economy with the DC on my own. Conversely, a DH who was grateful for me taking on more of the shit during travel and could see my point of view - even if ultimately we couldn't afford it - would mean I would be far more likely to agree.
People will disabilities can be selfish arses too - there may be a medical need but it doesn't stop DH from being considerate or appreciating what he's asking the OP to do. And I say this myself as a personal with a genetic, life-long disability.