Yes, he cut his hair, and the top too, because that was longer as well before the mohawk.
If you listen to the commentary from Ellie, they talk about how they can ask Eric to do a scene 15 different ways and he'll nail it every time, Gary compliments him as well..and he says that he was the one that cut Eric's hair.
And he did a pretty good job too, I liked that little mohawk, especially when it grew out a little..he had to do that though, those lines in the side meant he would have had to shave the whole thing, and if you look really close, when he first got it, those are still a little visible even with the mohawk.