My boyfriend was dishonest with me (20M) (19F).

I've been in a relationship with this guy for eight months, and he has truly been the best boyfriend I could ask for. He only focuses on me, showers me with love, flirts with me, and is incredibly patient when I have my moments. Plus, he’s dedicated to his studies and building a career; he really embodies what I would call a "man's man." Despite all the positive things I've experienced with him, there are some concerns that are troubling me. Recently, someone informed me that last year, my boyfriend allegedly harassed a female classmate and was in another relationship, neither of which he mentioned to me. When I tried discussing it with him, he flatly denied both claims, saying he was never in a relationship and that the girl was lying about the harassment because he rejected her. Now I’m wondering: should I trust him? The person who shared this information truly cares for my well-being and wants to help me avoid a toxic relationship. However, he has also told me things in the past that turned out to be completely false, including made-up stories about our mutual friends. Another point of concern is that my boyfriend often comes across as selfish; he tends to brag a lot, even about things he doesn't actually own, and he often lies to show off. At this point, these traits haven’t impacted our relationship, but I worry about how they might affect us in the long run, especially if we get married and have kids. I'm serious about him, but is this behavior sustainable for the future?