I really need an external perspective on this without ending up being bashed by people thinking I'm BU to the ExW/DSS.
DH and his ex have a difficult relationship but I don't get involved in general. I leave it to DH to make the arrangements with her to see his DS (my DSS) who is nearly 14. The contact arrangements are just a family arrangement, nothing court appointed and DSS generally comes over on a Sunday, but sometimes on a Saturday to stay overnight and sometimes other days in school hols.
ExW has always been a bit controlling and would often not let DH know what was happening until the last minute, which pre-covid although frustrating was manageable. But more recently we have had our DS (9 months) and obviously having to deal with the restrictions that lockdown has brought. (Pre-lockdown we would often go out for food but obviously now we eat at home) This means that I have felt that it would be helpful to have a bit of notice about what was happening in order to make plans, particularly for food (in our house I deal with most of the food/cooking related things as DH is fairly useless at that! 
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DSS is quite a fussy eater, only eats very limited veg and no fruit and is fussy over other things. I am pescatarian so when I cook a meal for me and DH we've agreed that we eat the same (he'd have meat with a takeaway or eating out) and we eat a lot of vegetarian stuff, lots of veg, salad etc - which DSS obviously won't eat. Hence me needing to plan! I am happy to buy things he likes and include meat for when he comes but don't really want it taking space just in case (our fridge and freezer are small) or ending up being wasted.
DH ex increasingly hasn’t let DH know until the last minute what the arrangements are, what meals DSS is here for etc, and sometimes expects us to drop everything to accommodate.
Much as I try to not get involved, I am getting very frustrated and have said before to DH that ExW needs to stick to a deadline (eg we need to know by Thursday or we’ll make other plans) but he’s tried that with her before and it just results in her nose getting out of joint and then she makes excuses for DSS to not come over for several weeks, saying he’s busy. And I don’t want to be responsible for DH not seeing his son.
Also, much as I realise it is not DH fault for these last minute arrangements, I am also annoyed that it is always me who has to organise the food situation and run around like a crazy person trying to sort something. If I ask what to get for DSS, DH just says “I don’t know” and I think “if you don’t know, how do I?”
I am honestly at the point of saying either ExW needs to give us sufficient notice for me to sort something or DH is in charge of the meal arrangements.
I have never ever wanted to create a “not my child” situation as I feel we are one family (we don't even refer to the two boys as step brothers, they're just brothers). I love DSS very much and want him to see his dad and baby brother, but at the same time I don’t see why I should be the only one impacted by the situation. DH will just tell me when DSS coming and expect me to sort something.