It is pretty obvious you love your son very much and are doing everything you can to make his life as good as possible.
It is also pretty obvious that your ex is a completely different type of person and by the sound of it is using that love you have as a weapon against you. After all it is probably the most effective one he has. It causes you great greif and as such he would see himself as being successful
Yes, extreme cruelty.
I can't answer why some peple are cruel, it is a fact of nature in the same way as some are born empathetic loving and kind. It just is, though that can be had to believe about someone you once loved and thought good.
I'd like to ask two things, the first being do you have proper legal advice, or have you been winging it using your own judgment? It does take an expert to successfully lead you though the maze of complications on separation.
The second is about your own welfare. While I'm glad the test came back negative that is not the only concern, as you are in a highly stressful situation. Under such circumstances I've needed both medical and family support (for a different matter)
So may I ask do you have medical support, if not a long consultation wiht your GP setting out the details could be of great help.
The other thing is who you can talk to frankly and share the burden. Now is the time to lean on someone who cares - it makes a surprising difference to the load, as I found out by confiding in my wife who became a tower of strength and common sense.
Can you talk to your dad or someone else?
Please let us know how you are getting on