If his suspension had ended then the rules attached to it had ended too.
Plus I imagine the other kid needed their trainers for the weekend?
Unfortunately you can have a kid with the wrong behaviours In the wrong school. Some are nurturing with bret pastoral care who realise children acting out are communicating something.
Some just want well behaved robots. These zero tolerance schools contribute to the high numbers of children with MH difficulties we are experiencing. I say that as a pastoral support worker across schools for pupils with SEMH.
These are 2 separate issues.
Your sons behaviour, the route cause and supporting him to behave correctly by identifying that and seeking support.
If true that the HT is building a case against him that is a separate issue. If - for example - they found out he was on the street with peers at 3.45pm and school finished at 3.15pm (and suspension has ended or the school hours curfew) and she then went and got a camera to take a photo to use it against him it could indicate that they are trying to find all sorts of evidence against him (when he isn't doing anything wrong at this point) and seek to off role him. This is illegal but it happens. I've seen it.
Have you ever read about managed moves? These can be so good for pupils in this situation. If your feels the school won't give him a chance to get it right he'll struggle to manage his own behaviour more and get it right. And let's face it - a kid struggling with their behaviour isn't going to suddenly become a perfect angel. They need realistic targets and chances. We want them to become fully functioning and decent adults and need to help them with this.
It's a chance for your ds to move to another school on a 6 week trial. He'll have targets and if it goes well they will take him on role there. It's a fresh start. It's used for pupils at risk of exclusion to prevent exclusion.
If you decide to go this route then look for a school with great pastoral care, a good send unit, that doesn't practice zero tolerance (he's clearly not in the correct frame of mind to be able to manage detentions for things like forgetting a pen when his energies will be focussed on managing his emotions) and definitely I would avoid any school that's ofsted outstanding (usually these are the ones who don't do pastoral well!).
I'm not a great fan of excluding pupils from contact with peers during fixed term exclusions. It can be counterproductive.
But I'd definitely learn from this that meeting them away from school is highly recommended!