Urg, texture effects, not my strong suit. Well, this is my best guess.
First, another layer above the artwork. Then create the basic texture, using: a texture file, one of Photoshop's brushes, or using a custom brush. I'm guessing he used a custom brush to paint the texture. Once you're happy with the texture pattern, set the layer's blending mode to Multiply (or whatever effect you're looking for) and adjust opacity accordingly. Looks like maybe he used more than one color for the texture layer, or used two different texture layers, but it's tough to say. This kind of thing is, I think, largely a matter of trial and error until you're happy with where you end up. Sorry if that's not particularly helpful, but it's all I gots.