He says he's not ready for a relationship. What do I hear?

“I'm not ready for a relationship” states a present limit. It does not contain the additional words “with you,” and it is not a promise of readiness later. The offer you can assess is the one being made now.

If you want a relationship, that difference matters without your needing to discover the hidden reason. A reply can acknowledge the limit and state your own direction; it does not need to persuade him to change it.