Giving Claptrap a high five

How do I give Claptrap a high five for the "Up High, Down Low" achievement? I found Claptrap in Sanctuary after Roland's house and it had a "high five" dialogue option after I finished its mission, but that doesn't do anything. Is that where I do this? Some place else? What's the trick?

When you get to Sanctuary, he gives you a quest where the reward is to access his stash. Once you complete it, you'll have a temporary option to hit a button (Q in the PC version) to give him a high five. He'll raise his hand.

At that point, hit your melee button (V in the PC version) to attack his hand. This will count as a high-five.

Be warned, though: if you're playing co-op and someone else completes the high-five, Claptrap doesn't allow you to request another one. :(

Edit, since this is the accepted answer: There is at least one other opportunity that has been identified by other answers on this question as times when Claptrap will ask for a high five. Here's the full list:

• After "Claptrap's Secret Stash" quest
• During the "Wildlife Preservation" quest

In all cases, the tactic is the same: When he holds up his hand, use your melee attack to hit it.

On a side note, once I'd given him the high five as part of "Wildlife Preservation", I could request one at any time. Even if you don't give Claptrap the high five in "Wildlife Preservation", as long as you complete the mission you can request the high five at any time. Aim at his hand and press V

There are two opportunities to high-five Claptrap (by melee attacking his hand):

1. After his Sanctuary mission, where there is a "high five" action (missable)
2. Claptrap's birthday party (roughly 8-10 hours into the game): The mission where you give Claptrap the upgrade will make him invisible. He will be happy and say, "High-Five! That's what I'm talking about!" while raising his hand. BUT, you have to melee his hand immediately because he'll get bored: "Uh... This is weird. Hurry up and slap my hand! Gimme a five! Okay, this is boring and awkward..."

For the record, having completed the game (thus missing both the opportunities outlined above), I earned the achievement by talking to him and melee-ing his hand. If it means anything, I did this while he was in his little junkyard alcove in Sanctuary, and I tried again when he left it, but could not do it again.