Mozza, I know it's hard, I'm struggling with similar myself, BUT...
The advice on here has been that you talk to them Adult to Adult, and mutually draw up a meal plan,decide who's going to do the shopping, cooking, clearing up etc.
You are saying yes, but YOU are still doing the organising ( Wifework) yourself. You say that YOU drew up a menu plan, you have called DP separately to explain what you are doing, YOU are going shopping to buy the ingredients DS wants. That is not relating to them as adults, that is still treating them all like children, and like it is your job to make sure everything happens the way you want. This is counter productive and will lay you open to to accusations of nagging.
It is 2017, you don't have to do the shopping yourself, there is the wonderful invention of online shopping, which is particularly great for those who don't drive.
I don't have it all sorted by any means, but we do now create a menu plan and subsequent shopping list together, and then one of the DS does an online shop for the whole family, to be delivered at a time to suit those who are not working, who will be at home for the delivery and who will then unpack it and put it away before the workers get home. If it's not on the list it doesn't get bought, except by anybody who wants to pop to the local shops themselves.
This works in so many ways - it means it's not solely your job to be devising meals that everybody will eat, it's up to them to think of things and to compromise. They include at least an evening meal each to cook. DH still doesn't get involved with this as much as I'd like, but as he's working quite long hours and is very appreciative of any food put in front of him, I don't mind when I feel that everyone is pulling their weight.