My son is trying to put a hole in a cinder block wall for a dryer vent. He drilled a number of holes in a 4 inch circle and was able to pop out the concrete, but the cavity in the block is only about 3 inches wide, so these are evidently blocks with more than two cavities in them. What is his next step? Can he just take out one side of the cavity to make for a 4 inch hole?

Might add that this is a two story wall.

No problem doing that to just 1 block. Look at a wall vent, they take out a whole block!