He mumbled I love you, could he be sincere?

This guy I'm "friends" (for 3 mos) with, with no benefits saw me yesterday. He's told me time and again he doesn't want a relationship. He has a bad habit of mumbling things or retracting them once he knew I heard them. He put his head down and mumbled what sounded like "I love you" when I asked what he said, he said something else. Then, for a second time he did it but said "I love you but I'm single." Why do you think he said it like that and made the last remark.
He mumbled I love you, could he be sincere?
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