The song is "For What It's Worth" by Buffalo Springfield.
It is one of 23 songs in the movie Tropic Thunder. During the movie, it plays when Ben Stiller is playing with his "son".
Here's the complete list of songs in the order they arppeared in the movie. The videos and downloads for them are at the link below.
I Love Tha Pu**y - Brandon T. Jackson
Gothic Monsters - Theodore Shapiro
The Four Seasons Spring Allegro - Capella Istropolitana
*** On Feel the Noize - Quiet Riot
Brick House - David Way
U Can't Touch This - MC Hammer
Sadeness Pt. 1 - Enigma
Ball of Confusion (That's What the World is Today) - Temptations
Escalation - Matt Hirt
Ready Set Go - Ben Gidsjoy
The Name of the Game - Crystal Method
Run Through the Jungle - Creedence Clearwater Revival
The Pusher - Steppenwolf
Sympathy for the Devil - Rolling Stones
Music from Star Trek episode Arena - Alexander Courage
Movin' On Up - Janet Dubois and Jeff Barry
Sometimes When We Touch - Dan Hill
Low - Flo Rida featuring T-Pain
Do Me! - Ricardo Bell, Michael Bivins, Carl Bourelly and Ronald De Voe (Bell Biv Devoe)
For What It's Worth - Buffalo Springfield
Frankenstein - Edgar Winter Group
Get Back - Ludacris
The Name of the Game (The Crystal Method's Big A** T.T. Mix) - Crystal Method
Original music for Tropic Thunder - Theodore Shapiro
Extra songs on the album, not in the movie:
I Just Wanna Celebrate - Mooney Suzuki
I'd Love To Change The World - 10 Years After
War - Edwin Starr