For all practical purposes, you’re responsible for his debt after you marry. In theory, you can partition off ‘pre-marital debt’ but the resources allocated to pay it off represent an opportunity cost associated with not moving other elements of the marriage forward.
Any assets you (or he) amass during the marriage would need to be moved to a trust to protect them from creditors.
A spouse can go buy a Range Rover on Monday, a BMW on Tuesday, a boat of Wednesday, etc. and you’re on the hook. A divorce, in this situation, is the only remedy. Unfortunately, in a marriage, there’s no mechanism to physically stop a spouse from spending (short of having the spouse deemed legally incompetent, with a guardian of his/her finances appointed).
It’s, frankly, a shame that you cannot separate the legal ramifications of a spending spouse (even if I’m a manic state of mind) from the true covenant of marriage. The ways to protect oneself include trusts, incompetency proceedings, divorce, or never marrying in the first place.