Yeah, for 1974 I don't believe they would even consider that a fifth grade kid would have bipolar. The realization that children can also have a mental illness is a relatively new one, even though obviously kids have always had symptoms or issues. It just wasn't recognized and even now I think a lot of doctors are cautious about diagnosing.
But, I say that since he severed contact at that young age, he probably didn't expect you to be there once he was diagnosed. I say not to be hard on yourself. It's just something that happened. Sometimes we lose a friend and there is no answer. Sorry about that.