Lots of replies there - and it's given me a bit of perspective
I think my issue is a combination of things.
I like to control what goes out of the kitchen. I am the one who shops - and I need to know what is left. I work all week, and have limited time and energy for re-stocking the basics.
Also - l tell DS what he can and can't eat. He is isn't allowed to help himself without checking with me first. I usually will say yes, but I just need to know he isn't using up the last of the ...... whatever. I also sort DH out, so he knows what is available. For example if I am going out and he is cooking I will tell him what I have bought in, and what I would like him to use. This makes sense so that things don't go out of date also.
The matter of him being step child probably does come into it - we pay his mother maintenance to feed and clothe the DC. But if he is eating the stuff I have bought in, then in effect we are paying twice ( and yes, I understand that it's only a few ££ per week...). And yes, he is entitled to the FSM - it is 6th form college.
We aren't struggling for money - but I am quite frugal - as I see what the day to day bills are , and the costs of shopping bills are getting higher and higher. I see no harm in shopping smartly - I don't treat me and DH to fillet steak every day - I try and keep it to a low budget as often as possible.
With regard to bulking out the sandwiches with salad - no go. He won't touch anything like that. I would do it fir myself - and I fill up DS' sandwiches with slices of cucumber to make it more satisfying.
Will have to think this through - and discuss with DH.
I guess I just don't like not being in control.