It's not a real song I think. I believe it is a studio-made song. It only lasts about 25 seconds. The song immediately following it is called "Lets Get Together" by the Youngbloods during the "Wave Scene".
It's not listed at all on the music used in the film page on IMDB. It's not a real song. Sorry man. It is cool though. Try downloading "Expecting to Fly" by Buffalo Springfield, written by Neil Young. It's good.
The Doc
And you liar, teller of tall tales: you trample all the Lord's commandments underfoot, you murder, steal, commit adultery, and afterward break into tears, beat your breast, take down your guitar and turn sin into a song. Shrewd devil, you know very well that God pardons singers no matter what they do, because he can simply die for a song.
| 
