Background 1: I'm going to be traveling to my home city in a few weeks with my husband for a relative's milestone birthday party. As such, all of my immediate family and the majority of my extended family is also going to be in Home City at the same time, and we'll be spending a lot of time with them.
Background 2: My husband is currently in the end stages of a PhD. He's currently writing his dissertation, and has made some definite progress, but he still doesn't know exactly how much longer it's going to take. For a variety of reasons, he's feeling a lot of pressure to finish it as quickly as possible. Added to that, he's also recently gone down some less-than-fruitful paths in his writing. He feels like he's wasted his time, even though he stopped and changed direction as soon as he realized that what he was doing wasn't working. Essentially, his frame of mind regarding his dissertation is not particularly sunny at the moment, and probably won't be until he's actually done.
Background 3: My parents, a few of my other relatives (the ones that have always had a quasi-parental role in my life), and my husband's parents have all been kind of harping about my husband's dissertation. They all frequently make comments that make it clear they think he ought to hurry up and get it done. These comments come from a place of caring and love, so we mostly try to ignore them (which is generally pretty easy to do), but they grate on both of us. They add pressure that my husband doesn't need and which doesn't help, and frustrate him because he
is finishing his degree just as fast as he can.
The Situation: Since DH and I will be spending several days with my family, particularly my parents, I'm trying to think of ways to gently but forcefully (got that?

) change the subject when DH's dissertation comes up. I know my parents mean well, but neither of them has ever done anything even remotely similar to a PhD, so they really don't get that it's supremely unhelpful to remind DH that he has to finish soon.
I don't really want to explain why it is so unhelpful, because that's more detail about the inner workings of our marriage than I think they ought to be privy to. I also don't want to shut them down too bluntly, or change the subject too abruptly, both because it totally would not work (it would make it into a Thing, and they would insist we satisfy them on the subject), and because they really are nagging because they care. It's one of the ways they show (unintentionally) that DH is a part of the family -- he's close enough to them that they feel a vested interest in his successes and want to help, not realizing that they are in fact doing the opposite.
My role in all of this is firmly in the middle. I would like to try to shield my DH from the worst of it, and figure out things to say that will allow me to change the subject without turning it into a Thing. Everyone in my family (myself included) can latch on to certain topics or ideas like a dog with a bone, so I'm trying to think of ways to avoid having that happen with DH's degree.
So if you've read all of that, my question is basically, how do you change the subject when you're talking to people who don't let go of topics easily, but who are also people you love deeply and don't want to be blunt with? I will get blunt if it becomes necessary, but since it would be at least a little unpleasant for everyone (DH and me included), I'd really like to avoid it.