What’s the best way for me (F19) to address my boyfriend’s (M25) sudden silence?

I'm a 19-year-old woman currently in a long-distance relationship with my 25-year-old boyfriend. We're approaching three months together, and this is my first serious relationship. In his dating profile, he described himself as “quiet, unless he’s with friends,” among other traits. He caught my attention, so I thought I’d give it a try. We clicked and ended up talking for hours every single day. However, recently—about a week ago—he started to seem a bit off. He wasn't mad at me, but he seemed generally annoyed. Despite that, he still laughed and smiled during our conversations. I tried to give him chances to share what was bothering him, and he eventually opened up about feeling dissatisfied with his work performance and thinking he could do better. Over the last week and a half, though, he has reduced his engagement during our calls. While he’s in a neutral to good mood now, like before, it’s confusing because he still wants to call every day for just as long as we used to. The difference is that he’s much quieter now, almost as if I’m not even there. I attempt to spark conversations, but his responses tend to be brief. He still compliments me and asks about my day, but something feels off. We're still texting, sharing memes, and communicating just like we did before; it’s just during our calls that he’s more subdued. Is he pulling away, or is it something else? I’m starting to wonder if my insecurities are getting the better of me. Any advice on how to navigate this situation would be much appreciated, especially since I’m new to dating.