I suspect that this sort of behaviour is so much a part of his life that he doesn't even see it as being wrong.
Please - whatever you decide to do (chuck him, I hope, but that is your decision) don't EVER give him access to your money in any way, shape or form, and don't EVER bail him out financially - not even a loan.
He took it for granted (in the same way his mother did) that you would subsidise the Newcastle trip when it became apparent that it wasn't a real gift. He didn't think twice about borrowing money off you so that he had cash in hand to pay for stuff, and didn't look embarrassed by asking you publicly. He may well pay off all of his debts - but he will borrow more because he has NO IDEA that money needs to be budgeted.
I think both he and his ghastly mother just spend what is there, take what they can, go into debt when there is no cash, borrow, beg or steal whenever they feel the inclination - they just don't see anything wrong or inappropriate in this.
Yes - he's had a shock, but that's because he (and she) feel that spending other people's money is their right, but that their own is sacrosanct - THAT is what has given him a shock. That the lavish abandon he applies to using other people's money has been turned on him.
His mother has taken out a loan. She has no assets (otherwise why would she be stealing from her son?). She'll fall behind with the payments - probably pretty quickly - what then?
She'll come crying to her son, you're DP. He'll be guilted into taking the payments over and he'll end up paying EVEN MORE on her loan than he would have lost had he just written off what she had stolen.
He doesn't dare go back any further in his finances because he fears it will irreparably damage their relationship - it's already irreparably damaged. But actually, I can't see him getting any more money back off her, so I can see why he might want to remain in ignorance.
I wonder what she has used as security for this loan. Will she end up having her home repossessed? Because I can tell you now, that with this attitude to his mother, your DP will definitely end up picking up after her if he is in a position to do so. Perhaps using your money.
I know you say he is kind, generous and supportive - so are many con-men. At best he is naive to the point of infantilism, at worst he is an amiable, charming rogue.
Please think very carefully about your relationship with this financial infant.