Can you screen shot everything, then send it all in a text to your ex saying something like " I'm not sure if you're aware of the strange messages your wife has been sending our son but I think you'll agree they're very inappropriate and are now upsetting him. If they don't stop now I'm going to look into legal advice"
Dunno about the legal advice bit!
I would be upset too OP. What a nasty manipulative woman!