Struggling to feel love?

My boyfriend [23M] and I [22F] have been together for five years. While we've experienced our share of highs and lows, one issue that's been growing is his belief that he doesn't know what love feels like and that he's never truly loved anyone. He only recently came to this conclusion. When we first started dating, he used to journal a lot because he found it difficult to express his feelings and understand them. For our one-year anniversary, he put together a book filled with his journal entries about me. In his personal journal, he wrote beautiful things about how happy I made him and how much he missed being close to me. He often shared how being with me brought him immense joy, even during mundane moments. However, things have changed since then. He stopped journaling and feels as though he's lost touch with himself. He claims he's never experienced love before and that he must not have been thinking clearly when he wrote those entries. Doesn't that sound unbelievable? Surely he has feelings, doesn't he? I could really use some advice.