Student or Learner
1. I've known him since childhood.
2. I've known him from childhood.
Do these two mean the same thing?
If you want to be clear, just say "I've known him since he was a child", or "I've known him since I was a child", or "I've known him since we were children."
No, I've never heard the implication that Fillet suggested.
In fact, "I've known him from childhood" is probably not grammatical; it requires 'since'; just as "I know him since childhood" requires 'from'.
"I know him from childhood" implies that you knew him when you were both children, and that you lost track of each other until now? And 'I've known him since ...' means you've known him continuously since childhood.