You are going to need to move. But to get to the point where your husband ends up agreeing with that, I'm afriad you are going to have to be willing to come down hard.
Change, right now, the way you engage. So they're coming down lots. You need to start showing them how it WILL be when they move. Firstly, get a ring doorbell so you can see who it is. Then- don't answer if you don't feel like it. If they call - sorry, I wasn't dressed. Be out. Be busy. Don't apologise for it. Then, when Words start to be had, you look astonished and say 'But, you're going to be living here now. It's not like a visit. We're all busy. We can't stop what we are doing every five minutes, as if it's the same as seeing people who live an hour away. It's going to be very different.
And you stick with that line. Yes they will get upset. They will grumble. And you smile sweetly at your DH and say 'Well I did tell you it would be a bad idea them moving here. It's so much nicer to be able to go for proper visits to see family, to have nice times - not have them breathing down your neck, when you inevitably both never live up to expectations.'
I agree by the way that this might have been discussed with your H, even if only in passing, as a 'we're getting older... we'll need help' thing. You MUST get out of this - you absolutely have to be moved, or them moved, before the question of care comes up. Because guess who is earmarked for doing it? That's right - you - which is why your H would not have breathed a word to you if they did speak to him in this vein.
So. Starting now, you make sure that their needs, wants, issues - are HIS problem. They want help with shopping? No problem, call DH. Always have an excuse, always have something else you are doing. When pulled up, look baffled - 'But I wouldn't want to get in the way of your relationship/it's your son's place surely/I know you'd want to be the one to be main carer for your parents, of course' (baffled look)
Hold this line and pretty soon, the thought of his aged parents next door won't make your DH feel all warm and happy that it's allll sorted out, but will give him the knot of anxiety at the thought of years of running around after them that most WOMEN end up being the ones to enjoy. And if it comes to it, you tell him straight. 'No, I am not going to be a carer for your parents. I was never consulted about them moving here in terms of care and if you did have that in mind, well you should have checked it out with me - that ball is FIRMLY in your lap, and I made it clear I was not happy about the move.'
Start as you mean to go on, right from a quick hello and sorry we are busy, see you soon - the next time they come down.
And when it's sunk in with your H, he'll be only too happy to house-hunt.