@Maldin's answer is very nice. I'll just chime in to answer your question, which one is preferable and why. I'll warn you though, this is pretty subjective.
He's at lunch
is good because it rolls off the tongue without having to think about it a lot, and it works in all cases -- he went to a restaurant; he went to the deli for take-out; he's having a salad at his desk and isn't answering the phone, for better digestion and stress relief; he's having a lunch meeting with colleagues; you name it.
Here's a nice situation for the present perfect. Someone calls for Jeremy. You say
Let me check if he's still here. (Calls over to co-worker) Has Jeremy left for lunch yet? (Into phone) I'm sorry, he's already left for lunch.