You are a fucking ass-clown.
Firstly, because the surest cure for teenage drama is to quietly let it blow over. For heaven's sake, this is /r/Parenting and your best idea for helping a teen is to have the police spend a solid week interrogating and threatening every single staff member and student, after which the panicking staff* will spend six months shooting about sexual perversions against the boy. And then the lawyer that you so idiotically recommended will restart the whole thing as a tort lawsuit. If you practice running, I bet you can win the Special Olympics.
Secondly, he has PTSD because the body responds to a heart attack by using every last molecule of adrenaline, at the same time his brain was dying of lack of blood flow. If he had his heart attack in a quiet room with a calm psychologist and velveteen animals, he would now have terrifying flashbacks of quiet rooms, friendly counselors, and toy animals. The treatment for fear and flashbacks is to confront them with the best mental therapy the scientists have found, not putting him into a parade of stressful situations. Running him down the legal gauntlet will make things worse.
*The pedophile paranoia in the UK is astonishing. This is the country where a mob found out that a man was a pediatrician and ransacked his office, just in case that was the same thing as a pedophile. Involving the police will start a year-long circus with OP's son as the star attraction. They won't run his name in the news, but the students will know. Oh, they will fucking know. He'll spend a year under the microscope.
My recommendation: Avoid the police. Make sure the school knows that his future welfare is what you value, not legal action for past misdeeds. Consider appealing to the other students to support him, to take action against anyone tormenting him. With a little luck, the other teens will be so busy sanctimoniously helping him that they won't remember to bully him. His friends may take it for granted that they support him and not realize he needs to be told that, so a little promoting might help.