OP needs a bigger freezer
Seriously the narrow minded view of so many on this thread beggars belief!
She has neither room nor money for a bigger or additional freezer - she's said this. I don't either, not everyone lives in homes where this is possible
@SingToTheSky thank you - and exactly! Being capable and independent really boosts their confidence and self esteem which this lad could really do with from sounds of things
My dd had her moments of thoughtlessness too - that is typical teen behaviour BUT we don't as parents have to accept it. This is when you point out to them that they have been thoughtless and inconsiderate and not to do so in future plus natural consequences (not getting to eat same as op and dh in this case)
I regularly had as many as 5 extra for dinner as that was the size of her wee "gang" she was close to - not a problem as long as either I had enough notice or she did the cooking which she was perfectly capable and happy to do. It was usually very basic/cheap fare as I was on a budget, plus I'm disabled so I can't spend ages standing in the kitchen making a risotto - but tasty nonetheless - pasta and tomato based sauce, bean chilli, stovies, casserole if I had enough notice...
The other families also fed the "gang" as a group they seemed to sort of rotate among the homes dependent on which parents were home which days which was dependent on shift working, what clubs/activities other siblings and parents had etc such that it settled into a bit of a routine eg I'd tend to get them on a mon and maybe a thu as that was when the other families were out/busy, dds BFF family tended to have on a sat as that's when that mum had her parents and in laws over for a family dinner and they had a big family anyway (4 kids, plus parents each from big families) so that was when she'd do a big batch cook of something, another family did a pizza night on a Friday and bought in pizzas from a local takeaway (cash rich time poor) and that was their way of connecting in a positive way with their children and their friends...
You get the idea, also yes I was on a tight budget but 4/5 nights a week I wasn't feeding Dd so it kinda balanced out anyway
Byproduct of this was also that all the parents knew all the dc fairly well and that closeness/bond has other benefits too - like spotting when there were additional stresses that kids may not mention to parents, like the dc pulling together at times of general stress like exam time. One of the "kids" recently called me rather...tiddly on their 21st birthday and said that birthday reflecting had led to them remembering fondly these times and how lovely it was to be welcomed at their friends homes, made same phone calls to the other families too I heard, it was sweet and amusing and much appreciated. They'd made a point of mentioning memories specific to each household.
I have ocd myself so also don't cope with last min changes which dd and her friends understood and they usually gave me enough notice.
You're so right that where possible (and your child having medical conditions that impact learning and possibly confidence?) it's to their benefit to teach them these life skills.
As a youth leader in various guises we sometimes had members with learning difficulties and if anything this made them more determined to learn and keep up with their friends, we also had physically disabled members - there are usually workarounds that can be done, the balance of skills across the group negotiated etc the first time I was in a group with a wheelchair bound member I was a little thrown with one badge but the child themselves cheerily went "I can do that seated, I just need x and y adjustments/tools to help me" she had a great attitude. Stayed in touch with her and her family after she aged out and she went on to have a great career as a mentor to newly disabled children.
I don't have a problem with stepmum teaching him, but it seems the dss and parents do and I can see in this situation if ANYTHING goes wrong even the slightest burn or scratch and the op will get it in the neck from all 3 which she doesn't deserve and slight cuts and burns are normal when learning to cook - that's why I think it should be on the dh
I agree the dh also has some serious growing up to do
I too am bemused by why op even got involved with such a family/man I'd have run a mile!
or a cheese toastie ( worth buying a machine if you don't own one)
No machine needed - I have a very small kitchen (floor space 1m x 1.5m) and I use toastabags - in which many things other than toasties can be cooked, eg fish fingers, bacon, veggie burgers, onion rings, chicken nuggets...
In the toaster as is - potato waffles, tattie scones (I can't remember the last time I cooked tattie scones any other way, frees up frying pan space 
)