Should I be worried that my son's the smallest in his class? He's 5

No need to worry. Boys can suddenly grow up a lot during puberty period and besides it is still too early to tell if he will be small. Even if he is going to be we still should accept the fact and most importantly is he is healthy :)

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There is nothing to be worried about. They will grow and next thing you know, they shoot up real tall and double in size. As long they are happy and healthy it's very important!

Nope. They will grow and spike without you realising. By then they would have really shoot up in height and double up in size.

Genes also plays a part in size. Most importantly is healthy!