First thing you should do OP is get yourself a copy of your credit file. Then you'll be able to see for yourself whether you're financially linked to any of this.
I would be taking the cards from him and get him on a basic account and transfer what he needs daily. Receipts for everything and I mean for everything. He's clearly poor with financial managing, so I'd be looking to take the responsibility of doing this from him. That said, the fact he's gone to great lengths to hide it from you (although you suspected something wasn't right) suggests there's a sneaky side to him that needs some serious thought from you on how you tackle him.
I say this with some experience. I hid some eye-watering debt from my family some years ago. I ended up driving myself into the ground, and doing some quite horrendous acts to keep it hidden while struggling on.
So as someone that lied, hurt and damaged people I love, deeply, even though now I've made amends that time for them was awful and for me too. It's not pretty or nice and your DH is contrite now, just be careful. If its a genuine horrible mess, without any other gross offences to you, then you may be able to come through it. Together & stronger. If he's just an utter cunt and is leading a double life, I'd prepare yourself with legal advice.
Take it one step at a time. Downgrade his bank account, get your credit report and begin.
Good luck. 