OK, so, I am guessing there is a lot of tone/backstory that says there's more here than meets the eye. Because it sounds like she is just assuming he is going back to work, because in her world That's What Men Do, and so my natural response would just be to say "yeah, ma, you know he's not going back to work, right?" and not get worked up about it. Sort of like if people assumed you were staying home, because it's more common that women do that -- you'd probably be a little annoyed at the stereotypical assumption but then just brush it off with a "well, actually . . ." and not make a big deal out of it. But, like I said, it sounds like there's history here with your mom in particular -- I'm going to assume she tends to be passive-aggressive and indirect and so conveys judgment through tone and careful phrasing. Am I on track here? (Not that, umm, I have a mom who does this at all . . . ).
So assuming unstated judgment, I think there are a few approaches that can work. One is the "reasonable grown-up conversation" approach: "You know, mom, you keep saying that. But you know DH is staying home with the kids, right? We've told you that, right? So why would you keep assuming something that's not going to happen?" Then listen. If needed, repeat that this is the way it is and it isn't going to change, and that it's not up for debate, and you hope she can respect your decisions. Etc.
Or you can try to draw the parallel with her own life so that she is allied in interest with your DH, a/k/a invoking the mommy wars on your behalf: "I had such an awesome childhood with a SAHM that we wanted our kids to have a SAH parent, and boy, DH just does an awesome job with them." Serving up some indirectness of your own can help with this, a/k/a in response to some article or program, "I hate it when people devalue the worth of the SAHP -- you were such an influence on my life, and I am so grateful we can offer that to our kids as well." And then there is always the highly passive aggressive fighting-fire-with-fire version: "I'd hate for him to feel like you don't see his role as important or valuable, because what he does is so important to me and the kids." [Note: I don't tend to do this well, but as long you can keep the irony out of your voice, this kind of subtlety can work extremely well, because it means *she* is the one who makes the connection between her life and DH's, and people are always more convinced by their own ideas (or the ones they think are their own ideas)]
The other option is to call her out on indirectness in general, but in a nice/funny way -- usually works best on non-emotionally-charged subjects, though. I started this with my mom as a teen, when the indirectness just got under my skin, so I chose to respond by being overly direct and blunt. That, umm, did not go so smoothly then. But now that I have been able to add a little humor and emotional distance to it, it works better -- e.g., I get a long-winded email asking if someone is going to be around between X and Y, and will we be going anywhere near the airport, and if so would it be convenient to drop her off at the airport; me: "you know, if you want a ride to the airport, you can just *ask* :-), and the answer is 'yes'." If you guys have this kind of communication and relationship, it becomes much easier to work in a direct response on topics that actually matter.
Oh: and these conversations are for you, not DH. Your job is to protect him from passive-aggressive crap from your mom.
The thing is, you're probably not going to change her -- sounds like she has some specific gender roles in her head, but it's hard to say how much of that is just unconscious vs. actively signaling disapproval. What you can do is bring that out into the open and give her a chance to see her assumptions in black and white, understand the impact on you guys, and try to catch/retrain herself to avoid that -- and then, if she keeps doing it, setting some respectful boundaries ("you know, mom, I'm tired of the passive-aggressive disapproval, so I'm just letting you know that if you keep harping on it, I'm done talking" -- and then hang up the phone, or (cheerfully) leave whenever that happens).