Both are correct grammatically, but number 1 is probably what you are looking for. It says that the boss is still waiting for the return call. Number 2, past tense, is saying that the boss had wanted you to return his call before. However, in spoken English, you will hear these sometimes used interchangeably. The way I would naturally write or say the sentence would be "The boss just called and he wants you to call him back."

Both are correct! They are interchangeable. However, fix the commas: Just now the boss called you up and, he WANTS you to return his call. --->Fix the comma: Just now the boss called you up, and he WANTS you to return his call. Just now the boss called you up and, he WANTED you to return his call. --->Fix the comma: Just now the boss called you up, and he WANTED you to return his call. Hope this helps <(^.^<)