I'd recommend against following legal advice from a free online forum, no matter how well intentioned.
In an ideal world, you and he would talk openly about the situation and consult an attorney together to find out what obligations each of you has. There are steps you can take together to firewall his debt and protect you, but they require a united front between the two of you.
I don't see a united front in your post. I see extreme secrecy on his part. One thing I learned about my RABF's secrets (and he was very secretive when he was drinking, still somewhat that way now that he's sober): it was always way worse than even my overly developed imagination could dream up. I found out early on that even though he made $90,000 a year, he couldn't keep up with his bills ("forgot" to file tax returns one year and the IRS garnished his paycheck). In 2008 he swore he had everything straightened out, and two months later I pulled up to his house and there was a foreclosure notice tacked to the door. He had gotten in late from the bar the night before and never noticed it.
My RABF had a huge amount of shame about all this, and that's why he kept secrets. I told him I had compassion for him, but I still had a right to know--either that, or he could tell me I had no right to know and I'd walk. To his credit, he didn't get angry and blame me for asking. The lies were bad enough.
{shuddering at the memory}