"1) I have recently started receiving abusive and very sinister text messages from someone claiming to be his new (and pregnant girlfriend) and silent anonymous
phone calls. He said at first he knew nothing about it but then admitted he WAS seeing someone but it couldn't possibly be her. Hmm. I changed my number
and didn't give it to him or his parents and the messages stopped! Given the nature of the messages I cannot say that my son would be safe anywhere near
whoever it was sending them."
Contact the police. You have the number of the texter, so it will be easy enough to trace who it is. But even if it is the girlfriend that's not necessarily reason to stop his contact - presumably he's not responsible for her? It would give you grounds to not want her to have contact with your ds, but not him. But you need to establish who is sending the texts before you start making assumptions.
"2) Since he left for the last and final time 7 months ago he has not paid a penny in child maintenance, in fact I have been supporting him - sending food
with my son when he visits and lending him money. He has also left me with a large amount of debt that he hasn't even attempted to repay"
That's irelevant. Maintanence is not linked to the contact with the child.
"3) He is shortly due to appear in court on a very serious charge, if convicted could face up to 6 years in prison"
If he is convicted then that will resolve your issue, but until he is convicted he is innocent until proven guilty.
"4) He has been under the care of mental health for some time now - claiming to be unfit to work and therefore to stand trial (yet is fit enough to commence
new relationships)"
There are lots of people who have mental illness who might not be fit to e.g. work, but who are perfectly able to have relationships with their partners/children.
"5) He is residing on a property with people known to the police for a variety of crimes"
If this is the case then you would possibly have a case for stipulating that your ds not be taken to that property, although again it depends on what the crimes are, if violent crime then yes, if petty theft, then it's unlikely the courts would consider them a risk to your child.
"6) He has, in the past, physically assaulted me and others"
A just because he's asalted you (which i am not condoning obv) doesn't mean he is a risk to your child. Plus as you've not used this as a reason before it's unlikely to stand up as a reason now.
"7) He has been seeing DS every weekend (almost), but I have had to make the arrangements and more often than not get my parents to drive my son to him
and pick him up. He does not contact him often in between weekends and often does not return DS's calls when he wants to speak to his daddy."
Then stop making the arrangements. If he wants to see his ds then he will realize that he will have to make the effort.
Look, I'm not saying the man isn't a twat, but most of the reasons given above would not stand up in court as reasons for stopping his contact.