I think you should permanently swap his phone for a cheap pay as you go that can only make phone calls / texts. No wattsapp / internet capable phone for at least a year, no giving the expensive phone back after only a few months: you tried that and it failed. Donate ‘his’ phone to charity: he has used it as a tool of bullying and thus demonstrated he doesn’t deserve it.
Permanently delete any social media accounts he has, and tell him bullies aren’t allowed on social media.
Call the school and ask how to discreetly pay for the blazer. Make him write a letter of apology that is much more than “I’m sorry” he needs to explain why he did it and if he doesn’t know why then he needs to write “I don’t know why I’m like this, I wish I was a better person” etc
Consider asking the school to suspend your son for a week, or at least give a load of detentions, so that other kids know he has been punished. While he is off, make him work hard so it isn’t free time.
Chores/general penalties at home are unlikely to change his behaviour as subconsciously he won’t associate it with the behaviour and will just feel ‘parents are mean’. I wouldn’t bother with extra chores doubt they will be effective.
Agree with comments upthread about needing to teach him empathy. That is hard but can be done. He needs to practice looking after someone/something. Volunteering etc. I’m not convinced viewing videos about the impact of bullying will help, he might just get a kick out if it.
Tell him if anything like this ever happens gain you are putting him in a different school to separate him from that friendship group. And follow through.
Research the causes of bullying and explore them with him and possibly a therapist. Does he feel powerless? Etc. It is interesting that he chose to talk to you about a ‘crazy kid throwing away his blazer’, seems like he wanted to discuss the incident with you without giving away his role. That means something psychologically I dunno what, ask a professional.