Step son asked me to promise not to tell anyone

I have 3 kids. Two of them are step kids and one is biologically mine but they're obviously all my kids. My step son is 27. He comes over at least once a week. I say we have always had a decent bond. He was smoking weed with my husband last night and my husband went to bed. We talked before he left and he thanked me for being a great step mom because his step mom prior to me was awful. He asked me if he told me something would I keep it between us. I said sure. I'm going to be honest. I thought he was going to tell me he was gay. Everyone believes he's gay including his mom. We have all just had our suspicions. He told me not to tell anyone and this includes his dad and especially his mom.. It wasn't that he was gay. He told me when he was 13 he was molested by a doctor. I didn't expect that at all. He told me the story of what happened. I don't want to trigger anyone so I won't say details but this seems like a big thing to keep from his parents. But he's also not 13. Had he told me this when he was 13 I would have told him I would have to tell my husband and his mom. He's 27. He's a fully grown man. I know you aren't supposed to keep secrets from a spouse and also are supposed to tell you step kids biological mom things. But again. He is 27. I feel like that isn't my place

Edit: I think why he doesn't want his dad to know is my husband is a very hands off person emotionally. Not just as a parent but as a spouse. He didn't grow up in a loving home, we don't talk to his parents, so he is very emotionally unavailable at times. It sometimes comes off as cold and my husband isn't a cold person and he does love our kids and I know he loves me, but how he is he can come off as uncaring because he's so emotionally guarded and hands off. The reason I even made the promise is because I thought he was going to tell me he was gay and my husband has always said if any of our kids came out to me and didn't feel comfortable coming to him to tell them he will still love them but he won't be mad to me for keeping that a secret if our kids ask. This is obviously very different.