I would cancel for this week, with an excuse that doesn't refer to this. That buys you more time to discuss with your son as to how you, as a family, are going to deal with this.
Our school's policy on bullying is that the parents should always talk to the school, NOT the child or the parents. We are from an ostensibly "naice" school, but there has been arguments between parents and one parent whose child had been bullied by another, took it upon herself to hold the bully by the ear and put her fingernail through it!
Whether you are good friends or not, not parent likes to hear that their child is a bully, and it's crucial that this is handled by a third party (ie the school) who can referee. Be prepared for the fact that this boy may make up stories to his parents about how your son is the bully and turn on the waterworks. It can get very uncomfortable. I have a friend whose yr 6 boy has been bullied mercilessly by a former BF, and his mum now blanks her and refuses to engage. It is really tricky.
I say all this, NOT to stop you saying anything, but just to ensure you do it the correct way. Get your son to keep notes of everything that happens - when, where, who was involved, what happened.
Please do reinforce to him that it's not his fault. I would also consider looking at some self defence classes or similar. It's not just about being able to protect yourself, but they are very good at building self esteem and often attended by other children who've been in the same situation.
I would also consider stopping the activity and changing it for something else, or if he really enjoys it, finding a different class. It will be very uncomfortable for him to go to an after school activity with