This sign is one step down the self-esteem ladder from perceptions of not fitting in and not belonging. It is almost all the way to the bottom.
There are a couple of points to consider. Most children and most adults for that matter sometimes feel unloved and unappreciated; and many feel a little like this much of the time. Their feelings may be about low self-esteem; but they are more likely because of some atypical circumstance. When feeling unloved is a sign of damaged self-esteem, it is much more constant, much more severe, and very painful.
Your child says, "No one loves me. No one cares." You say, "I love you. I care." You then go on to say, "Not feeling loved is a hard feeling to handle. I can tell it feels awful and hurts a lot. It seems like you feel angry and afraid and other confusing stuff at the same time. Not thinking anyone cares would feel really lonely. How does it feel to you?" At least your child now knows you care enough to care about how he feels. That is a start.
I love you's are cheap; but when you are gentle and honest, your child learns he matters enough to be listened to. This is a small bit of self-esteem, a moment when the most important person in his life respects him enough to deal with him openly and honestly.