Translate to english a veces no serio Es un chico?

The literal translation of "a veces no serio Es un chico?" is "sometimes not serious, is he a boy?".

However, this sounds very awkward in English.

To make it sound more natural, you could say:

* "He's not always serious, is he a boy?"

* "He can be silly sometimes, is he a boy?"

* "He doesn't take everything so seriously, is he a boy?"

The best translation will depend on the context and the intended meaning.