"The new boy"
It is not diminutive, those are different words. And I would say in meaning it is more like chłopak is augmentative form of chłopiec.
Chłopiec means "boy", but it is less often used for young people, who are not considered "children" anymore. Chłopak means older boy, young man , youth ; also boyfriend
I think that are almost the same level of formality, but in the formal situation, I think chłopiec still fits, while "młody mężczyzna"( young man); nastolatek (male teenager) fits better.
Also in plural "chłopak" is a tricky word, it has a declension that is complicated for Polish people. Plural nominative is "chłopaki" which needs not male personal adjectives and verb forms.