The kid with cancer shouldn't have been treated any different by the opposing team. Not playing your best, regardless of who your opponent is or what their situation, is by definition poor sportsmanship. Suppose the kid was just small, or couldn't hit, or didn't run very fast? No one would have a problem with him being struck out then.

I have nothing buy sympathy for a sick kid, but the fundamental rule of good sportsmanship applies here. Based on your post, it seems like the boy with cancer understood that.