Grammatical name and function of this sentence when he came back?

This phrase is not a complete sentence. It is a dependent clause, specifically a subordinate clause.

* Grammatical name: Adverbial clause of time

* Function: It modifies the main clause (which is missing in this example) by specifying the time when something happened.

For example, the full sentence could be: "When he came back, he told me everything."

Here, "when he came back" tells us when "he told me everything."