Sorry if you felt thats how my post read tryhard, not my intention.
I'm obvioulsy not aware of what he eats in a normal day, but my ds (6yrs) will have porridge and banana for breakfast, school lunch - he has fruit or yogurt for pud, a sandwich or houmous when he is home from school, then dinner as a family - usually home cooked and lots of veg.....so pretty healthy. So in my eyes, if he has a mini bag of haribo once a week, this is reasonable within his normal diet.
If he were having chocolate cereal for breakfast, lots sugar for lunch and dinner, then it wouldn't be ok.
I think because you give a generally healthy diet, the sugar won't hurt him on these odd occasions.
How you can handle this....hmmm, well i think I would do a bit of perception work for you both. So a few pictures of foods and drinks and then you both sit down and put them into everyday foods, a few times a week foods, once a week foods and not very often foods. So you can help him make choices when he is at a party. Difficult as you don't want him to develop hang ups about food.
Wound the school say sweets are ok, but they go in their bags til the end of the day and they have to ask their parents if they can eat them?
We've always allowed ds to have sweets and chocolate when he wants it and have found he rarely opts to eat it.
I hope you've received some support with eating disorder? I have no experience so I can't comment other than a big hug.