Filling a CMU block wall in garage
To insect proof the block, you only need to fill the last 4" of block. Stuff newspaper in the core down a ways to keep the concrete of masonry mix from dropping all the way down. 

Filling the whole core is not needed and to attempt to fill it from the top, through those small holes will only be frustrating. It will not go all the way down anyway. It will stick to the sides and other cross webbing you do not see and start collecting in places before it gets to the bottom.