For a 4 by 4 cube, he needs 4 rows 4 high for 16 cubes in each row.
The back row is 3 x 4 = 12, so he needs 4 more for the back row,
The 3rd row he has 8, so needs 8 more.
The 2nd row he has 4 so needs 12 more,
The first row he has 2 so will need 14 more.
Total needed: 4 + 8 + 12 + 14 = 38 more cubes.
