The most popular tables that DJ's use are the Technics 1200 series (MK2 or M3D). The M3D is the updated version of the MK2. They actually cost the same right now retail ($400 each). Can be found cheaper possibly online or through ebay. I'm not sure who recommended a Gemini to you but those tables in general are pretty low end from what I've read (never used one myself). Vestex makes some good tables too (PDX-2000). Numark's TTX1 is also a pretty highly rated table. All these will run you around the $400-500 range. I've mostly been keeping up with price on the Technics tables and I've found that used can be had for around $350, sometimes as low as $300.
You have to remember that after getting a table, you'll need a good mixer to do any type of performing. Can't do much w/o a mixer other than play the records and perform the most basic scratch. If you are going to skimp on the price somewhere, i would suggest doing so with the mixers. They are easier to upgrade in the future if necessary. Better than buying 2 new tables. You can get a good entry level mixer for $100-200 depending on features and type of mixer you want. There are cheaper ones but $100-200 will get you a good working one that may not need to be upgraded for a while. Stanton has a pretty good "scratch" series of mixers with their SK line. Vestex is a very popular brand that a lot professional DJ's use but you're gonna pay a premium for their mixers.
I'm currently using a Numark PPD01 mixer
Numark mixer. Cost me $400 at Guitar Center but can be had for around $325-ish on ebay (new). It's not the best for scratching because the cross-fader is a bit on the heavy side but it's great for mixing and recording. It has 8 differnt built in effects you can control with the cross-fader, dual mode tone control, and has 1/4", RCA, and optical outputs for a good amount of connectivity. I just recently ordered a Stanton SK-2F limited Edition mixer that I'm going to try out and compare to the Numark. The stanton has less features but is supposed to be better for scratching. The Stanton cost me $225.
Also, most tables do not come with needles/cartridges so you'll have to buy those too. Gets kinda expensive when you add it all up. I've been finding that ebay has some pretty good prices on all this type of stuff.
I am by no means an expert so any one that wants to correct me, please do. this is just based on my research the past month and a half. I just recently bought a set of turntables myself to do pretty much the same thing your son is doing: Mixing, scratching, and recording (sampling). So far, i've been real happy with my Technics SL-1200 M3D's.
www.hollywooddj.com has some very good prices for new stuff and does a good job in describing the different products they carry.
Well, i hope this helped a little bit. I'm currently at work so i'm just typing out some info pretty quickly. If you have other questions, post them and I can try to answer them. Again, these are just my recommendations and experiences i've had so far. Other people will have different opinions.