Where I am at common law isn’t recognized. If it’s his debt, it’s only his as you aren’t married but you need to investigate laws in your area.

I’d not consider it theft if he didn’t steal your money but if he didn’t disclose his financial habits, I’d call it being inconsiderate, not really stealing. Was he always this way? Was he financially irresponsible before you decided to have two children together? Or that was a sudden change?