@Shehasadiamondinthesky Your poor son you both have my sympathy. I've been through this myself I was bullied horrifically by my ex boss and ended up having a nervous breakdown and I took an overdose (thankfully I didn't take enough to do any serious damage) so I can understand what your son is going through.
Are HR and OH aware of his mental health problems? Assuming they are then they have a duty of care to support him in any way they can.
For starters if your son has a signed doctor's note clearly stating he is unfit for work then she cannot stop him from being paid. The only time SSP would stop is if your son has been on long term sickness i.e. 6 months or more and then he would have to claim ESA. Is there a way you could access their workplace policies and procedures? If so give them a read and it should state about SSP and sick leave.
I would sit down with your son and write down everything that has been going on. When the bullying started, what she's doing to him i.e. belittling and discriminating against him, creating a hostile work environment etc. Put every detail in there and date and time it as much as you can and write down if anyone else was present. Get a doctors note or letter from your son's psychiatrist (if he has one) clearly stating his current mental state and the cause of it. If she has phoned your son repeatedly then screen shot the calls list as well along with any text messages and emails if she's been threatening in those.
As for telling your son he has to have one to one meetings with her and he can't have anyone present is a lie. Due to the fact he has MH issues he is allowed to have someone present during a meeting and she can't stop that whatever she says HR and OH will back this up because they have a duty of care to all employees and provide any support they can to meet any needs including your son's.
You can intervene and handle this OP because if your son is not in a fit mental state to deal with this then you as his legal next kin can step in and speak on his behalf if he gives his consent for you to do so (I have this in place when I go through MH issues my mother is allowed to make decisions on my behalf because I consented to that with my manager at work when we put a plan in place to support my MH needs). Don't call the bullying bitch, in fact don't answer any calls from her from now on.
Call HR and ask to speak to someone senior. Tell them who you are and explain that your son is very vulnerable right now and has understandably asked you to speak to them on his behalf (they may want to speak to him briefly to confirm this is ok). Have what you have written down to hand just in case then tell them everything that has been going on. How she has behaved belittling and threatening him with not getting paid etc tell them how it has effected his mental health including him feeling suicidal and the possibility that he may be sectioned over it. Tell them about her phone calls to him which is stressing him out badly and that you feel she is harassing him whilst he is trying to deal with his mental health, then talk about how you were present for one phone conversation and how she spoke to him and what she said. Explain to them that as this is seriously impacting your son you feel you had to bring this to their attention and that you will leave this for them to handle for now.
Once you've got off the phone OP send them an email basically saying what you discussed on the phone something along the lines of "Hi X. With regards to our phone conversation about...please see the following concerns I raised to you about my son and bullying bitch behaviour towards him...If you need to contact me I have included my mobile number. Yours Sincerely Shehasadiamondinthesky" That way there's a paper trail of everything that has been discussed and there can be no misunderstandings but be prepared the bitch will lie and deny it all and say your son is the problem or he's misunderstood her etc. I guarantee you HR will shit bricks after getting your phone call and email she'll be in a meeting pronto over her behaviour and hopefully be in shit so deep that ten paddies with shovels won't be able to dig her out!
I would also make it clear that from now on that bitch is not to contact your son at all directly or indirectly and if her phone calls continue then you will report her for harassment. Explain that you will call HR directly with updates about your son as and when needed. I doubt they will argue with that after everything that has gone on in fact I'd be surprised if they didn't bend over backwards to accommodate your son.
Then take everything you have written down and all doctors letters to citizen's advice OP and get their advice on your son's legal rights, I've used them before and they were brilliant really helpful and introduced me to a decent solicitor who sent a letter to my ex cunt of a boss on my behalf and she was on edge for weeks afterwards she bent over backwards to please me actually LOL. Also ask citizen's advice if you should email HR all you've written down including sick notes/letters etc. I hope this was helpful OP good luck and let us know how you get on and hopefully your son gets some breathing space to recover 