One element is what cicero just mentioned. The mother doesn't seem that involved in DS's life. And then suddenly, she remembers the *stuff* (not her existing child, but his stuff) and wants it so she can save money.
I think another element is that it's such a pain and an inconvenience for you to get this stuff. My DH's mother died years ago. His father left all of his (father's) and his late wife's stuff and let DH have to sort through all of it himself. FIL just left and DH had to go through it, throw out what he didn't want, sort through what he did want, put it in containers, move it from apartment to apartment. DH was in college when his mom passed, so DH didn't have extra rooms in houses to store stuff. He kept a few containers of things like his mom's jewelry and some papers of family photos and genealogy.
Ten years later, numerous houses and apartments later, FIL said to DH "I need my old clothes." FIL had had some inexpensive, cheap quality, out of fashion, and by now far too small for him pants and shirts that he had expected DH to keep all these years. Even after DH told FIL many times "I have to throw your stuff out because I don't have room to keep it." When FIL finally said he wanted it back, DH told him for the umpteenth time that most of it had been donated because DH didn't have room to store it. FIL hit the roof. DH did some tabulation and showed his father that had he paid $150/month for a storage unit, it would have cost over $15,000 to store all these things. He asked his father if his 1980s and 1990s polyester clothing was really worth $15,000. Then, FIL said "Fine. Whatever you still have, box it up and ship it to me." So DH was still expected to crawl around the crawl space of his house and find the few sentimental items he'd kept so that he could send them to FIL. And pay shipping himself. One of the things that FIL had told him he wanted - same as your DH's ex, he didn't ask, he demanded - was his late wife's jewelry. So that he could give it to his new wife.
I remember saying to DH "If I married a man, I don't think I'd want to wear his late wife's jewelry!" It was all costume jewelry, so not worth any money if you resold it. We didn't have kids yet, but I told DH that our hypothetical future kids were going to have nothing of their late grandmother's. I told DH not to send all the jewelry. I said to pick out the things that he remembered his mother by, and send the rest if he wanted, but to keep some for himself and/or our future kids.
I was working 60-80 hours a week and in grad school. I didn't have time to help DH root around our crawl space, you couldn't even crawl on all fours, you had to commando crawl because there was so little room, to drag out any boxes. And DH didn't want to spend hundreds in shipping to send any boxes to his father several states away. DH finally offered to pay for a UHaul trailer and let his father come help him crawl in the crawl space to get what he wanted. FIL took him up on it, and ended up also packing and taking back some of my stuff that was in the crawl space, without my knowledge or permission (it was camping gear, so replaceable, but still expensive).
If you're feeling charitable, you can tell your DH's ex that she's welcome to come root around your attic and look for what she wants, if she wants it now. Or else, you'll get to it when you get to it. I think if you bend over backwards to accommodate her this time, it'll set the precedent that she can keep demanding things and treating you this way in the future.
If you do let her come crawl around your attic, hide things that are yours (not necessarily baby items, like camping gear) that you don't want her to take. And also set aside and keep (hide elsewhere) the baby things that you want to save for your own future children that you don't want her taking.
