I put in the actual date that the microchipping happened, but some people can't do that (i.e. animals from a shelter who came pre-microchipped). As long as you can prove that the chip was in place pre-vaccination (perhaps a letter signed by your vet stating that?), you should be OK. I think they would prefer an actual date and clear documentation of that, though, if you have it.
You don't always feel the microchip, but you should get him scanned again. My cat's original chip disappeared (likely moved behind/beneath a bone where it wasn't being picked up by the scanner), so we had to re-chip him when we started the process.
Good luck!
ETA: Is that prior to Jan 12 or prior to Dec 1? I suppose either way, that's more than 6 months ago, but it could be an issue if your vaccination wasn't after both those dates, as they want to be clear on the dates.