An excerpt from Wikipedia:
The origin of the title of the song has caused great speculation. In 1924, the first editor of Kiwanis Magazine, Roe Fulkerson, published a column carrying the title "He Ain't Heavy, He's My Brother". Dated September 1924, the article speaks of Fulkerson's encounter with "a spindly and physically weak lad" carrying a baby and "staggering towards a neighboring park".
" 'Pretty big load for such a small kid' I said as I met him. 'Why, mister,' he smiled, 'He ain't heavy; he's my brother.' "
Fulkerson goes on to examine his profound regard for that statement and how it could perhaps help us to view life in a better way.
The phrase is also associated with Father Edward J. Flanagan, the founder of Boys Town. Father Flanagan came across a line drawing in the Christmas 1941 edition of the Louis Allis Messenger, a company publication. The "Two Brothers" line drawing of a young boy carrying his brother featured on Page 44, in gold & black ink. The caption read "He ain't heavy Mister - he's m' brother!" It was created by Mr. Van B. Hooper who later became the editor of Ideals Magazine. The drawing was subsequently repeated in the first issue of Ideals in December of 1944.
(Inspired by Woody!)