I agree it sounds like your SS's toys are getting taken away by their mother, it also sounds like they are reacting to some of her distorted thinking ie: the plush toy/restaurant incident. SS sounds like he is trying his best to keep the toys his dad gives him but can't. Of course he can't because you can't be rational with someone who isn't rational. It's so sad.
My SO's uBPDxw has been both overly indulgent and neglectful of her daughters, she isn't capable of the hard work that is day to day parenting.
You are ringing bells for me regarding the items that disappear in to the black hole of the kid's mother's house. My SO's uBPDxw was always keeping the kids clothes at her house... .she rarely bought the girls clothes... .too mundane... .too boring... .too responsible. She'd buy makeup and jewelry when what these girls needed were boots and winter coats! My SO was providing everything (and still does). He would have to buy socks and underwear monthly on top of other items that went to mom's and never came home. It was like the Dryer "sock monster" gone wild!
My SO also bought his older daughter clothes for school only to find her mother wearing them. (need the eye rolling emoji here!)
The sad thing is that the uBPDxw was evicted several times and finally had to put everything in storage... .only to not pay for the storage unit. All of her children's things from before 2 years ago are gone now... .auctioned off.
In regard to buying things my SO's uBPDxw never took care of anything, never kept track of anything, lived like a horder, and if she couldn't find something or the kids couldn't find something it would just have to be bought again. She didn't take care of her things or the kid's things.
All we can do is to continue to model good parenting, healthy family behaviors,keep validating feelings... .you know, show them there is another way to live. Each time you visit with those boys you make an impact.
Panda39