Wiz this is like the situation my son was in. His abusive partner tried every dirty trick she could think of, and when the offer of mediation came up, he knew she would sabotage it, but he still went along. The important thing is to show willing, and he should attend for the preliminary meeting and be clear that he will engage in mediation, whether or not there is an unlikelihood that it will work. Ex-DIL didn't engage in the process, and continued to deny access, and my son gathered a thick file of evidence of her attempts at parental alienation. The judge took a very dim view, and my son was permitted to meet with his son, 11 at the time, and for them to define what access they wanted to each other, then at the next court hearing (which ex-DIL didn't show up for) the judge got ex-DIL on the phone and admonished her for her behaviour, and she was advised to agree the contact arrangements which the judge deemed appropriate.
Things are never going to be easy, but my son does see his son every week, sometimes several times. They come here for Sunday lunch and to see other family members, and in November he is taking my grandson to a convention (some geeky thing) in Disneyland, Paris for a few days, just the two of them. A year ago, we were in despair, worrying that we wouldn't see my grandson again, and we were denied contact with him over Christmas, but here we are now, back to some level of normality.
Keep to the rules, try everything you can think of, and record every single thing that is done to prevent access, then present it to whomever will listen. Organisations like Fathers4Justice and Families need Fathers are excellent, and they have worked alongside Cafcass to get some guidelines for access between parents and children accepted by family courts. Worth joining. Good luck!