Well, first, people can be disappointed about a pregnancy "scare", and can be "in" to kids and it doesn't necessarily mean they are really ready to have kids NOW. I want to say to not read TOO much into this.
While he may be disappointed you aren't PG, he might also be thinking "Well- it isn't the right time yet, so... maybe it's a good thing.".
But- to bring it up, I'd say to him "So- recently when we found out I wasn't PG, you said you were disappointed. WHere are you at w/ all of this? Are you beginning to think you want to start having kids sooner than 3 or 4 years?"
And let him talk! Find out where his head is at.
And if it comes out that he does think he'll be ready sooner, or even that he wants to start NOW, I'd probably say something along the lines of:
"Well, to be honest, as we've always said we're going to wait until our 30's, thats where I'm still mentally at. However, I'm not adverse to talking about this more and re-evaluating our timeline. HOWEVER - I need some time to think about this. I need a few weeks or even a couple months to really think about this and see where I'm at - then I'd like to revisit the topic. Can you give me that much? "
Don't go into it assuming you're going to be on opposite sides of the issue, or that it will create stress. If you calmly talk about it, if you CAN be willing to consider changing the timeline, that might be all he needs to hear right now.
And he might say "Yeah- I want to have kids sooner", but that doesn't mean tomorrow! He might be thinking in a year or two instead of 3 or 4 years!
But just gently bring it up, see where he's at and then just TALK. Be open to what he has to say, explain your situation, and hopefully he'll give you the same courtesy and will be open to what you ahve to say.