If your son is just getting started, you should probably stick with an "entry-level" or "student-level" set.
Typically, they are 5-piece kits that include a bass drum, two tom-toms, a floor-tom and a snare. Also included in these kits are a basic set of "sheet cymbals" (hi-hats, ride and crash), and the necessary hardware (i.e. cymbal stands, pedals, etc.). With exception of sticks and sometimes a throne (the drummers seat), these kits are complete.
You can typically get these sets for about $400, give or take $100. Almost all of the "big" guys make lower end kits. So, if he is looking for a name brand, more than likely they will have entry-level kits. (see Ludwig:Accent, Tama:Imperialstar/Stagestar, DW:Pacific, Pearl:Forum, Yamah Rydeen).
Check out www.musiciansfriend.com for the best prices. You can see almost everything they sell at a Guitar Center if you have one nearby. They may even give you a similar price.
It is true that these kits are not the best, but someone just learning does not need the best. He just needs something on which he can learn and practice.
If drums turn out to be his thing, he can begin to upgrade the set in the following way:
1. Upgrade the cymbals to set of profressional "cast-bronze" cymbals. This will provide a marked improvement in the overall sound of the kit. Cost: About $600 for a set of 3 or 4 professional cymbals (see Zildjian, Sabian, Paiste). These can also be done one at a time.
2. Upgrade the heads on the drums to professional heads. The heads that come with beginner sets are often the low-end heads. A good set of heads with a proper tuning can make most any drum sound good. Since heads are "wear and tear" item, if he plays alot, these will need to be replaced eventually anyway. Cost: $150 to $200 for new batter and resonant heads (see Remo, Evans, Aquarian).
3. A new snare drum. The snare drum is the most important drum in the kit. It will define the kit's sound more than any other one drum. Cost: $200 and up.
4. A new shell kit (toms and bass). Unlike student-level kits, when buying higher end drums, cymbals are rarely included and only sometimes include snares and hardware. Therefore, the previous upgrades will not go to waste. Cost: depends on the make and model as well as the number and size of the pieces (see Tama, Ludwig, Pearl, DW, Mapex).
5. Hardware. If the professional level hardware is not included with the new shell kit, it can be purchased one piece at a time to replace the aging hardware that come with the beginner set. Cost: depends on the piece. Professional cymbal stands can go for as little as $50 a piece and high-end drum pedals can cost $500. (see Tama, Pearl, DW for the best hardware).
The point is, don't spend much money on the initial set. As you can see a "pro-level" set can cost thousands. It would stink to spend all of that money only to have him get bored with it in six months. Selling them used will probably get you less than 50% of the cost.
If it turns out he's really into it he can upgrade as layed out here. Also, he can finance some of the upgrades by trading in or selling the older stuff.
Hope that helps. Good Luck.