Hey Barb,
Always liked that moment too, even more so on repeat viewings knowing the not-so-good-with-a-can-opener moment is to come. I agree with your idea that Jack's happy about saying something clever.
And he's happy about the fact that Ennis was responding to him, enough to give him a break from the 4 hour commutin and the smellin of cat piss.
No one, except Jack's mom and maybe Ennis' mom (we don't know about her except by inferring that Ennis got his gentleness from her), seems to have ever responded selflessly to our boys until they found each other.