Use a metronome. Anytime you hear boom, it's a down strum. Any time you hear da-da it's down-up. Any time you hear da-da-da its down-up-up. He uses down, down-up, and down-up-up in different combinations through out. Get a metronome and practice playing one down stroke, one down stroke and one upstroke, and one down stroke and two upstrokes all in the same time. Once you can get the timing of doing the different strokes each within a single beat, you'll be well on your way. As far as how to combine down, down-up, and down-up-up, I think that's going to be more of a feel thing. But if you try to play it that fast immediately, you will fail and you will continue to fail.
Also, you need to be able to make the chord changes smoothly and cleanly in time with the music. If you cannot do that, then you need to pull that metronome out again and set it slow and make the chord changes IN TIME. As you learn to make the chord changes in time, slowly increase the speed on the metronome and continue to practice. It may take thousands of times doing each chord change to get it ingrained into your fingers so that you make it in time with the song rather than in your own time.
Rhythm is FAR more important than accuracy of the notes. If you mess up the rhythm, you've messed up the song.
If you want to play fast like that guy, then you need to start slow and work your way up to it. You need the muscle memory. If you are trying to play fast like that right from the get-go, you will never, ever succeed at cleanly playing that song because you will be practicing errors in timing and chord fingerings. Do it only as quickly as you can do it cleanly and accurately. Accuracy first. Speed will come.
- Zurf
Granted B chord amnesty by King of the Mutants (Long live the king).
If it comes from the heart and you add a few beers... it'll be awesome! - Mekidsmom
When in doubt ... hats. - B.G. Dude