The correct form is: "Ich habe einen Jungen." (boy) The nominative "ein Junge" becomes "einen Jungen" in the accusative.
The (nominalized) adjective "der Junge, die Junge, das Junge" is not often used. But the form would also be "einen Jungen / einen jungen" if you refer to a masculine word. The form is mostly used elliptically, e. g.: "Ich habe im Keller einen alten (Wein) und einen jungen." In nearly all cases, the capitalized "einen Jungen" refers to a boy (in the accusative). The form "einen Junge" doesn't exist.