Being open and honest is the best way forward, but work out what you want beforehand and accept he may not provide it
In late 2020, during lockdown, I met a nice guy. We started dating and our relationship developed at a rapid pace.
Six months into our relationship, I found an email on his computer from his university sweetheart. When we first met, he had told me about her, and how, 20 years before, he was in love with her and wanted to marry her, but his family disapproved. She later got married to someone else, and he moved on with his life. The email was a short correspondence about general things, nothing “out of line”.