John Sebastian wrote the music for You're a Big Boy Now, including "Amy's Theme". Incidentally, it's not surprising that you picked up on the harmonica on that song. Sebastian's father was a classical harmonica player - https://en.wikipedia.org/wiki/John_Sebastian_(classical_harmonica_player - and some of the musical talent was obviously passed down.
John Sebastian lived in Woodstock for many years and, as far as I know, still resides there, so you'll have to take a bit of a road trip if you want to have a go at him. I met him once, and he seemed like a truly nice guy. I mentioned that I had seen him at one of jazz guitarist Eddie Diehl's gigs. His response was, "He's my idol." In addition to playing, Eddie Diehl was also a luthier, and did guitar work for Sebastian.