Honestly OP, you're worried about how to come through this together but he's not worrying about how this is effecting you is he? So you're not a partnership.
First, you need to stop trying to fix this for him.
Go through your bills and really look at whether your contributions are split fairly.
Do a credit check you so you have factual information on how much he owes.
Then sit down, tell him you know he's in more debt than he told you then be quiet and see what he says.
If he admits it, tell him you're glad he's admitted it but you're at the end of your tether and ask him to put a plan together to fix it.
If he denies it, hand him the credit check print out (or screenshots if digital) and leave it with him.
What I wouldn't do is give him 1 penny of the running away money or take on any further debt in my own name.
You keep rescuing him and he isn't improving so you need to decide where your line in the sand is. How many times are you going to do this?
To be honest, I could (and have) forgiven this once. I wouldn't forgive again - he's lying to your face, he's being secretive and dishonest, knowing his actions impact you.
Stop trying to fix it. You need to get angry and make this his problem to solve.