Until that changes, that person looks for someone who fills a void in them, and that's not creating real love and happiness.
What he really needs is someone who makes them happier and better about themselves, which may feel too uncertain for him as long not have reached the stage being ready for that. The so called void should first stopping taking the overhand in close relationships.
They need therapy, TBH. Attachment styles go back to infancy and take time and work to overcome. The avoidant attachment style never learned how to *be* loved and be safe and contented inside that love. They have learned since childhood that the need of affection and attention is the surest way to be rejected, so to compensate they develop a pattern of reassuring others and themselves that they are not "too needy" in exchange for some morsel of affection. And in many ways, this is how they have learned to love others, by being turned off by perceived neediness and too much affection because that was the pattern they grew up with.