A very frustrating situation for you and your man station4. I suppose what you have to bear in mind is that the courts act upon cases that they have had in the past and then pass judgement as to what 'generally' happens.
So, looking at past situations we know that some husbands batter their wives. We know that they are physically larger and stronger than women and in society as a whole, the male of the species is generally the wrong-do'er'. Just turn on to Crimewatch to see the ratio of male:female violent crime committed. So unfortunately, men have made their own beds.
This does not help your partner in any way of course. I have read a similar scenario to this on AB before and I'm sure it happens to alot of men.
The only thing your partner can do is to protect himself and cover his back. ie, Take on the advice of the solicitor, record all telephone conversations and have a small dictaphone hidden in his pocket when he sees his ex. He need not engage her in conversation and only talk about his son with her, no matter how much he is goaded.
Don't let the strain and the stress of this situation interfer e with your relationship, as this is probably her aim. It may also be useful to predict that the woman in question will speak badly about both yourself and your partner to her son. In which case you will need to reassure your partner's son that he is loved and safe and make sure that he doesn't think that it's all his fault.(Which is what children always think)