I assume it was a joint mortgage - with both your and your ex's names on it. If this was the case, then each of you was liable for the whole of the debt on a "joint and several" basis, so the mortgage lender could pursue either of you for the whole debt.
I don't understand what you say about a 50:50 split. You & your ex may have agreed this between you but I can't believe the lender agreed to it, as to do so would reduce their ability to be paid. If I am right on this and there has not been any agreement with the lender then your ex can still be pursued for the whole amount outstanding.
However, if I am wrong and the lender did agree to limit your ex's liability to 50% of the total then there is nothing you can do about getting him to pay more (assuming he is not willing to do so voluntarily).
A lot of debt collecting firms put pressure on debtors in various ways. If their demands are unreasonable in your circumstances and they won't accept whatever you can afford then it could help to get assistance from the local CAB, or a free advice service such as CCCS.