I am really sorry that your DS is being picked on; teenagers can be absolutely vile.
However I don't think you should send it especially as it seems like you haven't known the parents since primary and haven't met them and therefore won't know how they will react. It might make things worse, and that is too much of a gamble.
My DD was picked on mercilessly in school and I wanted to send a message such as yours every day. So I really do get it 🫂 But the parents might think the sun shines out of their kids' arse, and it might make it much worse.
One of the parents whose daughter was picking on mine did eventually reach out, after DD stopped speaking to this group. She wanted my DD to be friends with hers again. I said now was not a good time as DD was still really hurt by them all. She sent me an absolutely unhinged tirade, that her DD was kind, that my DD was overreacting (she wasn't!), that the bullying hadn't been that bad anyway (it was!). This was a mum I'd been friendly with since preschool! So I thought I could be straight with her and even then I couldn't.
I guess basically what I am saying is that you don't know how people react and that a lot of the time they'll be in total denial about their child so will be defensive. They won't want to collaborate with you to stop the bullying; and the child might double-down.
If it was me receiving the text I would absolutely speak to my DD and do something but it is a huge gamble and may very well make things worse. They might tell their kid about the text and then they'll also pick on your son for having his mum send texts on his behalf.
So best to go through the school and also you'll have to be That Parent who doesn't quit phoning in and ringing them until they actually act. Don't let them fob you off!!! Sending good wishes xxx