It's kind of hard to know what to think when it's not certain what he's disclosing. Is it something illegal he did? Something abusive? Did he use to sleep with men (not that it should matter but some people might feel ashamed or shocked about that)? Was he diagnosed with some kind of mental illness in the past? Did he use to be an addict?

Like, I have a hard time knowing how serious what he told you is. You say it's "not that big of a deal", in which case I have to wonder how much right you had to "know everything" about it. Like - your spouse actually doesn't owe you every single detail of his entire past, if he's not telling you everything and the consequences are that minor, it's not the same thing as a lie imo. But again, I don't know what to think because I have no idea how serious his behaviour actually was.