Agreed - he's just a bully who isn't getting his way and he's trying to intimidate you - don't take the bait.
Send him similar, cool, factual emails whenever there is anything that he needs to know - and keep copies of EVERYTHING, write up a note about EVERY bad interaction and keep a file.
If you can stand it, write up a factual weekly email note about the kids' school progress, but DON'T reply to any BS on his part.
He's a nasty human being and he's infuriated that you didn't just die and let him move on to his fantasy new blended family without a hiccup. WASs honestly are surprised when everything isn't peaches and ice cream.
