Initially, the boy was ranked 20th in a class of 45 students. After two boys joined, his rank dropped to 21st. So, the total number of students becomes 47.

To find his new rank from the end:

The total number of students is 47. His rank from the beginning is 21st. Rank from the end = Total students - Rank from the beginning + 1 = $47 - 21 + 1 = 27$th. Thus, his new rank from the end is 27th.

The correct answer is C. 27th.