Hard to say. He doesn't make it any easier by wearing black and hiding his hair under a hat so often.
From about 2006 to about 2012, his hair was pretty similar, depending on how recently he might have had a haircut. After that, he started to let it grow a little longer.
Since he had that photo shoot in 2011, I may just have to split the difference and slip this photo #2 in there as well.
There are a number of photos in that collection of 9000 that I've had to estimate dates for - hopefully I'm not too far off.
Thanks!
Kate