Re: The LOST Thread: Beware of SPOILERS!!!
Yeah I think it's obviously Claire's Aaron, because if it was Jack's baby blonde hair would be an improbable thing. It could be a red-herring, but I doubt it for this reason: The previews for this episode said another member of the Oceanic 6 would be revealed in this episode, and that person would be Aaron, as no one else was revealed in the Flash-forward that we didn't already know about. So it's definitely Claire's Aaron based on that.
Now, why would Jack be hesitant to meet the kid? Their are some possibilities, either Kate some how smuggled Aaron off the island without anyone knowing about it, including the Oceanic 6. And Kate told Jack he got her pregnant (This is doubtful, since it's unlikely Kate would implore Jack to see Aaron if she lied about it.) It's also possible Jack is avoiding seeing Aaron due to the circumstances around how he escaped the Island. Did Claire died? Does Jack feel guilty? Maybe Kate and Jack took the kid because Claire went insane? Maybe the Island became less hospitable, and a baby couldn't easily survive? A bunch of reasons possibly.
I don't know if Jack would have found out that Claire was his half-sister while on the Island, as Claire herself didn't know about it either, maybe they could have pieced it together. Another possibility is Ben revealing this information to them if he had it.
LOST has been really good this season, I feel the flash-forwards have breathed new life into it.