Be prepared for the meeting. Tomorrow will be hard for him, but go in with pointy elbows and facts.
He has missed a significant amount of school.
He is showing avoidance behaviours.
The trust has broken down between the school and him and they need to make that right or declare that they can't meet his needs.
They are ignoring blatant signs of neurodivergence, and in your posts alone you've written a lot that indicates it could be ADHD, and you would like their full cooperation to have this investigated.
He is showing clear signs of not being able to access a curriculum, because his support needs are unmet.
You're aware his struggles started at the transition from primary to secondary. This is a big red flag. It is a notable period where a change in responsibilities and expectations present in children with undiagnosed neurodivergence.
They're issuing punitive measures to a child who is really struggling and this is impacting his self esteem. They need to seek correct guidance for his individual circumstance rather than follow an arbitrary process that does not incentives attendance.
Forced attendance by way of having an escorts round school is likely to increase his anxiety and emotionally based school avoidance. - He's already told you he is stressing about this. His mental health is in tatters. He's displaying key signs of needing social, emotional and mental health support from the school.
Stating things like "well he cant access an education if he isn't here" is further placing the blame upon him. Why would a child want to go somewhere that they've been made to feel shame repeatedly by way of exclusion and suspension when there were clear signs that he was struggling with the transition and those needs have not been met and have now spiralled.
Email the LEA. State something like "My son has had over 15 days of absence due to emotionally based school avoidance. Under section 19 of the Education Act 1996 the local authority has a duty to make sure he receives a suitable full time education for him. Please confirm what provision will now be put in place."