A boy's nickname can be a shorter version of his full name, such as "Sly" for Sylvester. It can also tell something about his interests. For example, he may call himself "Slugger" because he likes to play baseball.
The best nickname for a boy is one that reflects his personality or a memorable event in his life. For instance, if a boy has won many games throughout his life, his family and friends may often call him "Champ" to reflect that. Some guys have nicknames based on their physiques, such as "Bear" or "Hercules." Also, it's not uncommon for people to add the word "the" before a nickname for dramatic emphasis, as in "The Bear" or "The Beast."