It might be in his brush pack, but basically he's using the brush customization tools in Photoshop. You can set the 'jitter' amount (think randomization) on anything. Size, rotation, shape, color, value, etc.
If you have Photoshop, there is an icon that looks like a can of brushes. It's in the upper left next to the brush shape and size. Look up a video on customizing brushes.
If you look at sophisticated traditional painting, there are usually layers of color under the obvious top coat. Look at works by Hopper (Nighthawks) for example. I find this frustratingly difficult to master. :)