An easy way you can do it is to create a Timer object that fires every second.
Inside the code of the timer you call
seekBar.setProgress(player.getCurrentPosition() / 1000);
Additionally, you may want to implement setOnCompletionListener
to get notified when the playback has reached it's end so you can stop the timer. The timer should be stopped either if the playback reached it's end, the applications finishes, or you stop the music manually (in response to a stop button, for example)