Weight Gain, Jem Finch style

This would absolutely work, if you did it often enough:

"Jem was worn out from a day's water-carrying. There were at least twelve banana peels on the floor by his bed, surrounding an empty milk bottle. "Whatcha stuffin' for?" I asked.
"Coach says if I can gain twenty-five pounds by year after next I can play [football], he said. "This is the quickest way."
- Jem and Scout Finch, in To Kill a Mockingbird by Harper Lee, Chapter 26.

Whole milk + lots of extra calories (healthy ones, even). Twenty five pounds in two years on a diet like that? No problem, even for a very active boy.