How if your son's speech and articulation? On par or better/worse than your older's?
With your son's good number sense, I'm wondering if the problem isn't related to speech. My oldest also skipped a number (seven I think) for quite a while, and he's now the math whiz of the school. He had lots of articulation problems that didn't really clear up until age 3, so the effort in making sounds probably made "remembering" the simple order more challenging. His memory is stellar now, by the way, for everything except spellings, as he struggles with writing.
Little brother had very clear speech from very early and counted much higher much earlier. I don't think the difference results from that dramatic of a cognitive difference, but from the advantages of mastering good speech early.
