OP, I feel such an idiot for not reading the thread in full, before making my suggestion of him giving you a credit card in his name, so that you could pay for his child's expenses without footing the bill.
I've now brought myself up to date, and in a way am relieved that you've found out relatively quickly, rather than letting him fob you off any longer, as to why he was failing to repay you for DSD's expenses, and to contribute to things for the new baby.
However, I am SO sorry that you were smacked in the face with the true reason for him being in debt, as gambling is, in my opinion, even worse than being an alcoholic.
Thank goodness you have a loving and supportive family to go home to, I feel sure from what you've told us, that you will be welcomed, cared for, and loved throughout the remainder of your pregnancy and beyond.
Meanwhile, please, whatever you do, DON'T make the mistake of thinking you can change him, or help him, you CAN'T! No one can, except HIMSELF! And remember, none of this is YOUR fault, despite what some of the nasty posters on MN have tried to make out!
I also think that if you ever get to the bottom of things, that you'll find this is not the first time he's got himself in a mess due to gambling, and that his family are fully aware of his problem, which is why he's ended up renting a house from family, as he's clearly not going to be able to get a rental or a mortgage with his financial background.
Finally, I'd like to send you a virtual hug, and some💐to cheer you up. Wishing you all the very best with the remainder of your pregnancy, and the birth of your baby.