For every person trashing her, I'd vet that if we were to hear both sides of the story it would be a very different picture.
OP, you need to step away. Do not go writing to her, contacting her and offering to buy cots, or whatever. Leave her alone. She is nothing to you and she would have a case for reporting harassment if you were to try oush your way in like this particularly as she has already made it clear she does not want any involvement from you.
If your son doesn’t want to be with her then your son needs to end the relationship and be firm about it. No back and forth, no engagement with any antics, and no trying to reconcile. Be clear with her that any attempts to harass or abuse him will be reported. If she smashes his stuff, report it to the police. If she tries to contact him to harass or verbally abuse him, report it to the police. If she approaches him in the street or turns up at his home, report it to the police.
Once the baby is born he needs to go down the legal route to get contact arrangements in place. He can ask the court to order a DNA test if she refuses and can apply for a declaration of parentage. He doesn't need a solicitor and can do it all himself, there are various organisations who can offer advice on the forms and the process.
There is a guide here on applying for parental responsibility, if he is on benefits or a low income he could get the fees waived
www.gov.uk/parental-rights-responsibilities/apply-for-parental-responsibility
Once he has this then he can apply for contact.
In the meantime he would need to pay maintenance even if he isn't seeing the child because its still his responsibility to financial support him or her regardless.