It looks like shedding to me. Rabbits often shed in patches like that. The new hair that’s coming out is the wool, it’s darker and softer than the guard hairs. If you part your bun’s hair someplace else on his body, you’ll see that his “under-coat” is dark. 

Sometimes buns shed so they get hairless patches. It’s still fine as long as the skin is smooth and not scaly or dandruff-y. The new hair starts sprouting within 1-2 weeks, as a fine peachy fuzz.

Agree with Bam. Looks like molting in chunks.

The molting will expand but the molting pattern might be less distinct. Agouti buns tend to get very visible molting lines though. It’s probably because their guard hairs are banded in yellow-ish and dark.

Buns shed in mysterious ways. They can look extremely silly periodically, like if they shed off all the long hair on their faces and only have a velvety stubble left. Many buns look positively mangy when they shed. It’s nothing to worry about as long as the skin underneath is smooth and uncompromized. You should brush him often though, so he can’t ingest too much hair. Hair can accumulate in the intestines and cause serious blockages. We’ve had examples here of that happening. You can brush or comb and finish off with a clothesroller to catch loose hairs so they don’t fly around all over your house.

Some buns shed all the time. Most have 2 big sheds per year (spring and fall), and smaller sheds in between. This year 2017 seems to have been particularly terrible shedding-wise, all over the world buns have been shedding like crazy. One shed barely finishes until the next one starts up. A shed often starts in the head area. It can pause and re-start later. It’s a continuous source of joy for the bun owner. (No, it’s not, actually.)