If he has to build 4 walls, each having 6 courses, the total number of courses to be built is 4 * 6 = <<4*6=24>>24 courses.
If each course has 10 bricks, then the total of bricks for the courses is 24 * 10 = <<24*10=240>>240 bricks.
The total number of bricks missing to finish the last wall is 2 * 10 = <<2*10=20>>20 bricks.
The total number of used bricks is 240 - 20 = <<240-20=220>>220 bricks.
#### 220