Another vote for not taking this lightly. I would certainly discuss it with his psychiatrist. If nothing else, it shows how he feels about himself, how he values himself. Which is pretty low, based on thinking others wish he wasn't born.
Your sig says your doctor hasn't told you what your difficult child's diagnosis is. How does he qualify for an IEP and other services with-o a diagnosis? Often that diagnosis is a key that opens doors to services. Many diagnosis's can also lead you to information about what is going on and how to help your child. While not everything is wrapped up in a diagnosis, they certainly do open doors. You might want to push your psychiatrist to give you a diagnosis if for no other reason than to help get supports at school. Just a thought.
in my opinion your difficult child has a lot going on inside and things he says while in meltdown mode can give insight on it. Sorry his is hurting so much he thinks his own mother would poison him and doesn't want him to have been born. I KNOW you wouldn't poison him and don't wish he hadn't been born, but he is hurting a whole lot and those things are clear indicators. Saying you think your mom would poison you is very different from saying you hate your mom. I can totally see how you might take it in that spirit as it is so far from what you would do, but in looking at it from a distance, it says a whole lot about how he feels about himself.
It really hurts your mommy heart to think he believes what he says, doesn't it? I hope his psychiatrist and therapist can help with these feelings.