at this age it's very much a battle between giving them independence and still wanting to control what they do with it.
They're not babies any more who you can take to the shops to buy sweets and crisps, you're giving them money of their own but still trying to control how they spend it - you just can't.
At this age you need to give them a level of freedom while at the same time giving them the upbringing to know right from wrong. It can be a delicate balance, because they do still need boundaries but at the same time they also need freedom.
Buying sweets and crisps is what kids do. There's most likely a shop close to your ds' school - I know my ds goes to the co-op with his mates (and another thousand or so secondary age kids) after school and they buy all manner of crap. And truth be told, if one doesn't have money then they sub each other, so even not giving him money doesn't hold the answers and will just make him feel excluded anyway.
he goes to the school canteen and now has the freedom to decide what to eat. All the other kids have the same, and they will exercise that choice. Just make sure there is healthy stuff at home.
The no tablets at bedtime is one which is more important, and punishments in the face of deliberate defiance are perfectly reasonable.
But if you've allowed him to have social media then the key to knowing he is using it responsibly is for you to have the password, not to restrict which devices he puts it on.
If you are this strict about what he eats etc then he no doubt lies to guard against an overreaction on your part. And they all seem to lie on some level at this age, about the most stupid things. But the rule in my house is that while he might be in trouble for a behaviour, he will be in ten times more trouble for lying about it.
We had an issue with Instagram not long after my ds started secondary. Over the summer he had asked me if he could have it and I'd said no, but he set it up anyway after he started secondary, and then got into some issues with online bullying by some other kids. But because he'd set it up without telling me and I'd explicitly said no he couldn't tell me about the bullying, so when he had a day off sick it resulted in anxiety over going back, and he couldn't tell me why. It was only when my xh looked at his phone (on my insistence) and discovered the instagram app that the penny dropped, and we were able to deal with A, blocking the offending bullies, and B, speaking to the school to deal with the issue, and C, have words about lying and the consequences that can bring.
It is possible to be strict without being overreactive iyswim, and at this age there does need to be boundaries still, and as a PP said, they need to learn at this age that if you don't trust them they won't earn future trust on the bigger stuff. but as parents you also need to learn what the levels are of freedoms which are reasonable and unreasonable at this point, and restricting sweets when out with mates is one of those things which can't be enforced when the child is given the freedom and the money to obtain these things for themselves.