Jack has seen Santa a few times already this year. He'll never pass up the opportunity, even though he knows that its not likely to be the real Santa, but they'll pass everything on. Also I think he just likes to talk to someone who's only interest is a list of what Jack wants.
About Santa Jack has it all figured out, he knows that Santa has a lot of PR work in December and can't possibly be everywhere at once, so he has helpers that do a lot of the visits for him. As for who these helpers are, I think he's going with some kind of clones, because they're obviously not elves.
On one visit Jack expressed to Santa the desire for an iPad, by which he means any iDevice that he can play Temple Run on, he's wanted one for quite a while but he's hell bent on getting one this year for Christmas. Apparently Santa assured him that if he was a good boy then he would get one. So now Jack is just the most well behaved boy that there ever there was. You can see him analysing his every move, trying to work out if its good boy behaviour. Jack is pretty sure that this was that real Santa.
It's a little annoying actually because I feel a little like now if we don't get him the device then all of this hard work will be for nothing, but these devices are expensive. Jack keeps going on about when he gets his iPad and so on. Santa, the giver of gifts, has made this deal and we're feeling bound to honour it. Curse these clones.