Re: Sax recommendations??
I can't say that I know too much about the Kessler or Antigua brands, but I would recommend LA Sax or the Winstons out of the brands you mentioned. I know that price is obviously important, but it really isn't worth it to buy a soprano for less than $1000. Overall, sopranos are a tricky business since the physics of the instrument and the air movement make them naturally out of tune across the majority of the range. Even with top of the line Selmers or Yamahas this is a problem, and it only gets worse as the price of the horn gets lower.
I was actually in the same position as your son is about 5 years ago when I got my soprano. I ended up holding out for a while until I could pay part of the bill and my parents were willing to come up with the rest. I ended up getting a special run of the Yamaha 475 from not long before they remade the 475's and the price went up. It has been a good horn for me so far; I play in two different jazz bands at the University of Iowa, and the only thing I've done different with it is buy a new mouthpiece to get better projection.
So, I would recommend that you maybe do the same and hold out until you can afford a better horn. It might also be the time to blow the secret on the Christmas present and talk with your son about it. If he really cares about having a good horn, he'll be willing to hold out for a better one.