I can totally relate to how you're feeling. My daughter experienced some bullying in primary school and it hurts. You would do anything to protect your child and the frustration of not being able to fix it can be overwhelming.
I wouldn't send the message. (And that's coming from someone who would absolutely type up that message!) I always like to sleep on things, consider what might happen if, before I act or react.
Like others have said, it could ignite things. Not all of the parents are going to be nice and react well to it (their kids are getting the vile behaviour from somewhere) and they might just mention it to their child who'll then taunt your boy, "Oh look, he's got mommy to send a message!"
In secondary school, their attitude may well change as they're suddenly the little fish in a big pond. It can be overwhelming. It suddenly becomes 'uncool' to play playground games.
They may well all be split up and may only be together for, let's say, tutor period, then go off to separate classes. Is it a large secondary school with multiple form entry? Either way, they'll all be mixing with new faces and will want to appear cool, not babyish, and taunting your boy is incredibly babyish.
I'd hold back for now and see how things go. By all means have a conversation with the secondary school and give them the heads up. I'm disappointed on your behalf that his primary school have let things slide and their lack of action has led to the bullying being allowed to continue.