That he has engaged her but hasn't already told about all his financial secrets means he doesn't trust her, since part of trust is speaking truth. In a relationship as intimate as engagement and marriage, if he hasn't told her about all his financial secrets, since your relationship is only sick as it's secrets, then he doesn't have a real intention to marry her in his heart. Their relationship isn't built on truth, but lies and or a series of omissions.

He should either tell her immediately about as many of his secrets as he can think of, especially the ones that would give her a reason to break up with him, or break off the engagement and learn to speak truth to himself and other people as a way of life.