So, what do you think?

"He went very quiet and was visibly troubled by what I said. He seemed very concerned that others in his life had felt the same way and had just never told him."

I think that this is the heart of it; past the phase of a new relationship, you start to see and be shown what doesn't work effortlessly in your relationship. He is probably capable of greater sensitivity and is having to confront his own shortcomings - not of ability, but of effort - which you revealed to him. You did the right thing in expressing your feelings, and you should expect a (gradual) change in his behavior as the relationship matures. If you don't receive this, it will continue to grate at you until the relationship is terminated.

If I am being over-sensitive, how do I improve this? Or can I go back to hiding my reaction so it doesn't cause tension?

I don't think you are, though a reasonable amount of time should be allowed between expressing your desire for improvement and expectation of a change in his attitude. Hiding your feelings will not work for either of you in the long term.

And if he is truly being insensitive (in word choice/tone - not intention) how can we work on that? How should I approach a discussion with him? Or should I just give it some time and let things sort themselves out (after all, in the big picture, this isn't exactly a huge issue or deal-breaker)?

This is not something you can directly fix, but you can encourage improvement. You might find it helpful to briefly and consistently state when you've been offended - not a heap of drama every time, just a reminder that his insensitivity puts distance between you. Another discussion is probably not necessary as long as he understands the problem, it's just a matter of his choice to break the habit. I can't think of a good way to reward positive changes (perhaps you'd notice a more thoughtful wording, and say so?), but that's something to consider as well.

If nothing else, I hope this gives you some grounding to know that this is a reasonable thing to be upset by, and your feelings are understandable.