There are a few possibilities:

1. He does actually feel something, but he is not willing to share it or show it - he may even be overcompensating & therefore coming across as rejecting.

2. He doesn't actually attach to people. Like, that's just his personality. He understands it enough to play at it, appear one way, but does not genuinely attach or really understand attachment from a personal level. This is how S is, and it's been quite the learning process for me. I've been doing some reading on narcissism and attachment theory.