Firstly, to the poster who said, "My Ds is very sensitive too (has ASD) and it boils my piss when he's told he's a victim because he's an easy target." it's absolutely true that he is. Bullies look for mentally or physically weak targets. They look for someone who won't fight back. That isn't telling your son he's to blame, it's examining the motive of the bullies. I think you need to differentiate the two.
To the OP.
I'm a headmistress. Not always correct but at least have experience.
You're right to ask for help as this is a minefield. Heavy handedness from teachers, students or parents can make this much more difficult than it needs to be. I say teachers as although it happened out of school hours, we can and like to become involved. There's no separating what happens outside the school boundaries and its effect in school.
I think you should approach the school. Not the other two boys' parents as you don't have a relationship with them, good or otherwise.
The school should be aware of the situation. They should handle it correctly. They won't turn your son into a grass and will ensure that this doesn't escalate.
I wouldn't call this bullying as that is usually repeated occurrences. This doesn't make it any less of an issue but the sensible option is to look to repair the friendship as opposed to a creating a simplistic 'perps and victim' scenario and keep the children apart.
At this age, children can be nasty and often are. As someone else said, there's a world of difference between a sustained campaign and a one-off incident, however unpleasant that incident was.
Take photographs of the blazer just in case it's necessary to show evidence.
WhitePhantom The school won't march up to the two boys and say "Jendall's Mummy said that you've been mean to him...". The OP needs to trust the school. They know what they're doing.
I realise this is the hardest part but remember that they're all only children. These 3 may become wonderful friends with the other two completely changing as they mature. You might be seeing a lot of them over the next 7 years or so.
Your husband is in the wrong (of course) although it's good he cares. People live vicariously through their children and this isn't necessarily a bad thing. As well as the upset that he wasn't able to protect his son, he'll also feel upset his son couldn't defend himself.
I rarely think teaching bullied children any form of martial arts or self defence is a good idea. It can be a great sport / hobby but not when the idea is to use it in a pre-defined circumstance.
The most upsetting and difficult time I've had in my career in education was when a bullied child (we, with the parents we working very hard but it was a sad and difficult case) was sent to boxing lessons despite our suggestions to the contrary. The child was 10 or 11. One day he was pushed too far and thumped the bully. A single punch which dislodged 3 molars and knocked the bully out cold. As they fell to the floor they broke their nose and fractured their eye socket.
I ended up in court defending the bullied child.
I realise this last part is fairly OTT and unusual, but people suggesting defence classes should think through the possible repercussions.