I agree with Dan in regards to the old Learning Company titles. He's probably at the age where he can use the Rabbit series without a problem and benefit from it educationally, but is approaching the age for the Treasure series or possibly even the Super Solvers series depending on his current academic ability.
Some older Learning Company titles offer more differentiation than others. In general, the math titles offer the most. Math Rabbit has "quick sets" for certain grade levels. OutNumbered can be tweaked quite a bit; one can even choose the numbers to appear in drill problems (useful if a student has mastered, say, n * 1 and n * 5 but not other times tables). A few products will present progressively harder problems as the player progresses, which is fine, but can't be reverted to easier problems if one struggles (Gizmos and Gadgets, the science program, is a good example of this). Gizmos and Gadgets can, however, display only certain types of problems (useful if you want to focus on a particular subject, such as force or simple machines). Some of the products offer no user customization at all (including Midnight Rescue, a reading program with multiple choice questions about passages which can be helpful for standardized test preparation). User-generated content can be used occasionally, such as in Spellbound, where a customized spelling list can be entered.
Another factor to consider is your son's hand-eye coordination. The Rabbit series is pretty easy to use--all you need to do is point and click. The Treasure and Super Solvers series, for the most part, require far better "gaming" skills. This element adds to the fun of the games for most, but for those who maybe have never played a video game, moving the character around on the screen and involving him in the action needed to win the game can be frustrating. I once observed a six year old struggle with Midnight Rescue because of the controls (he was fine with the reading aspect). Basically, you need to move a character around with arrow keys (or the mouse, which is harder for most) and be able to both zap robots and leap over/dodge obstacles. The Treasure games are a little easier in this respect than the Super Solvers titles (the one exception is Spellbound, which has no action components but does have its share of puzzle games).
The voice in the programs is clear and authentic--no C3PO wannabes here. It's more natural than the speech from OS 9 and definitely beats the original MacInTalk. Not every program makes use of speech, and Spellbound only speaks certain words (others will be flashed on the screen). Another innovative feature of the non-Rabbit programs is the online help in the form of "new user messages", which are displayed the first time a player goes through the game. They can always be turned on again later if need be.
Finally, if you are going to purchase these for an LC, make sure you avoid the CD-ROM versions. These usually require more horsepower than your LC has. The floppy-based versions will work fine. Keep in mind there are several versions of Reader Rabbit 1. I recommend v3.0, which comes with color graphics and does not require the original floppy every time the program is launched (I also have v2.2, which is in monochrome and needs the floppy). Gizmos and Gadgets claims to require a 68030, but does run on the original 68020-based LC.
On a hardware-related note, what monitor are you using with your LC? If you've got anything larger than the 12" RGB (512 x 384 resolution), you'll definitely want to stick with System 6. Some of these programs run in windows and it would be extremely easy for an inexperienced user to return to the Finder inadvertently with a poorly-placed mouse click. A few do allow for this to be hidden (Kid Pix comes to mind), but for now I'd recommend single Finder unless you have a program requiring System 7, in which case it may be wise to either partition your hard drive or keep two versions of the system on your drive, using a program such as System Picker to select the version you wish to boot with. (Do not install two SSW versions on the same volume together without a System Picker-type program).
If you wanted to use At Ease, you would need System 7, but I'd recommend against that unless you're seriously short on RAM for whatever reason (it uses less than the Finder; I've used it to maximize 2MB systems in conjunction with the DiskTop DA, which acts as a Finder substitute). However, since he's got to learn the ins and outs of the GUI, he may as well start with the older Mac--many elements have not changed much from System 6 to OS X/Windows 7 and he can gain familiarity and confidence with the processes of file management as a result. Make sure to teach him about backing up data at a young age!
If you can get a mic for it, you should definitely 
