It seems that the difficulty you will have is persuading your x2b that it is in his interests to withdraw the petition. You don't have to share them with us, but think long and hard about what's in it for him as these will be the persuading factors.
If your argument wins his support it's simply a case of encouraging him to instruct his solicitor and for him not to assume his solicitor's advice is instruction to him.
If he has already spent quite a lot on the petition you might have a difficult job though.
My x2b has been talking this week about withdrawing her petition as she is angry that her solicitor is taking so long. She has talked about me then petitioning her through an online service. This probably won't be an option for us as she is so changeable, but maybe you have a more amicable arrangement?