Megna portrayed Dill (Charles Baker Harris) in the 1962 film masterpiece, To Kill a Mockingbird. Dill is Jem and Scout Finch's summer neighbor and friend, and he represents the perspective of childhood innocence throughout Harper Lee's novel.
He would reappear on my own personal cultural radar in season one of Star Trek, in the episode "Miri," originally broadcast on October 27, 1966 (two days after I was born). Though credited as "Little Boy," many no doubt think of him best as being the "bonk bonk on the head" kid.
IMDb entry, he appeared in several more minor TV and movie roles throughout the '70s and early '80s. Most significantly (in my humble onion), he portrayed the "Young Hyman Roth" in The Godfather Part II (1974).
After "cameo" roles (putting it nicely) in both Smokey and the Bandit II and Cannonball Run, I guess John figured the acting game was up, and he became a high school English teacher, last teaching at James Monroe High School in North Hills, CA. On September 5, 1995, in Los Angeles, California, he died from AIDS-related complications (damn, you hate to see that) at the age of 42 (source).