Right, so your mum loaned you money. You were aware of this, but your DP was not. You had the money paid into your DPs bank account. Did you ever discuss where that money had come from? Did he know that it was a loan?
Are you currently making payments? Were you making payments before your mum took you to court?
Are the payments all that you can both afford, or just you? When you send your income sheet, did you include his income?
From what you've said, he seems to believe that this was a gift from your mother, whereas you imply that you knew it was a loan all along.
Your partner could attend mediation, but as he is denying that the money was a loan, he wouldn't be expected to agree a payment plan. You could, as you are accepting that it was a loan.
Your defence here will be interesting because you could undermine each others' cases. Essentially, you are saying that it was a secret loan that you had paid to his account - did you have plans to pay it back? Was it intended to be paid back in one go, or in payments, and if in payments, are you paying the correct amount or are you now paying less?
His defence would be that he wasn't aware that it was a loan, and wouldn't have accepted it if he had. If this goes in his favour and he can prove it satisfactorily, he may not be liable for further payments. That would mean that you were liable for the whole amount. In reality, it'll hit you both anyway - he'll be expected to cover more of the household costs so you can make repayments, if you have separate incomes, and the payments will be calculated from your household income if not.