Indeed. That's also evidenced from the fact that Sam didn't even know about hunting until 1991, when he was 8, and read John's journal without permission. John needed Dean to know because he needed Dean to know the basics on how to keep both himself and Sam safe while John was away - but Sam didn't need to know, so he wasn't told. Since Sam rung that bell himself, we don't actually know when John was planning on letting Sam know - but it's clear that it was a choice to keep Sam innocent for as long as possible.

Part of me thinks that John must have known that everything hinged on Sam from fairly early on - because he did isolate them from the hunting community. Most likely, because he knew that there were hunters like Gordon out there, who would hunt Sam just for being connected to evil, even if Sam had never done anything wrong.