It's funny how on a thread about the evil of bullying, some of the comments towards OP are leaning towards exactly that...
OP your son needs a severe wake up call, but his actions at 13 do not automatically mean that he is going to be a terrible adult. However, you need to act now.
I would speak to the school and offer to pay for the blazer as a starting point. Make it clear that you don't condone his actions at all, that you are taking the matter extremely seriously and will support the school in whatever punishment they deem appropriate. This is what the parents will want to hear.
Then at home really let him have it. Phone confiscated for a good while (don't sell it, it may cause resentment that could make matters worse). Ban on any other devices in the home that can access social media. When you eventually let him back on, make it conditional on you monitoring his use regularly.
Make him pay you back for the cost of the blazer.
Chores chores and a few more chores. If that was what he hates most, then this is exactly what he will get.
He's not going anywhere other than school, even when lockdown is lifted.
The biggest telling off you and your DH can muster, including how bullying can effect people, how disgusted you are with his actions and if he ever does anything like this again, he'll wish he'd never been born.
Stay firm and hopefully you can turn this around. All is not lost just yet.