MS or ME? Don't want to sound unsympathetic, but if it's ME, he's being a complete dick, if it's MS, he's just being dick.
I've had a massive spinal injury that ruined my surgical career, left me paralysed below the neck pre-emergency surgery, and I have shed-loads of metalwork in my neck and spine. I'm also 6' tall so not hugely tall but not short either, although I'm quite slim. My spine, especially my neck, is very stiff, poorly mobile and very painful.
I have never suggested, and would never accept, sitting in business class while my wife and kids sit in economy. I would see that as the height of selfishness and couldn't even contemplate it for a second before dismissing it.
As an aside, I've also done a fair bit of business class flying, particularly transatlantic. European flights are usually pointless with the exception of one or two specific airlines which you'd be unlikely to use on holiday, e.g. business class flights on Turkish Airlines to Istanbul are great whereas BA business to anywhere in Europe is NO extra legroom and simply a blocked middle seat plus a crap meal. My favourite transatlantic carrier is Virgin, and Virgin Upper is far superior to BA equivalent. You can get much cheaper carriers if you shop around and change routes/airports, e.g. Aer Lingus, Delta, etc., but with children, I tend to find that speed rather than comfort is of the essence.
Just my 2p worth.