OK. The Boss BR-532 Digital 4-track, for $399...
...records to 32MB SmartMedia memory card (included), expandable to 128MB. 4 primary tracks +8 virtual tracks. Onboard effects and stereo drum track. Record digitally and dump data to computer or CDR recorder. Sounds good, right? The only downside to
the BOSS BR-532 is that it appears to only have one input, and only record one track at a time, which would be a hinderance to more advanced recording techniques. It is all digital, though, and for a low-end product, it looks pretty good.
-There's also a KORG PXR4, digital 4-tracker. It has one 1/4" input, a stereo mic/line in, plus built in mic, and appears to record up to 2 tracks at a time. The Korg PXR4 also records to smart media cards, like the Boss BR-532, but appears to be much smaller than the BR-532. The Korg PXR4 retails for $399, also.
If you're determined to go digital, right out of the box, then these units would be fine to start out on, especially if your son likes super-high-techy things.
On the other hand, for the "bang for the buck" and pure, out & out simplicity, for $399, I'd recommend skipping over the Boss, the Korg, and all the digital gizmos, and go for the Tascam 424mkIII 4-track cassette Portastudio. By far, you get a much more full featured and robust production tool with the 424mkIII, than you do with either the BR-532, or the PXR4. I'll put the BR-8 on that list. IMO, the Tascam 424mkIII also blows away the BR-8.
The Tascam 424mkIII, for $399, has a much more complex and powerful integral 6x2x2 mixer, and by far, a simple cassette based unit is much easier to work than the digital workstations.
When all is said & done, you may still mix your 424mkIII/cassette recordings onto your PC, into Wav files, and burn CDs of your work.
Also, if your budget figure is appx $399, you'll barely scratch the surface of computer-based recording, for $399.
Again, if your son's a 'natural', hands-on home recordist, he may like the simple analog 424mkIII Portastudio format better, 'cause you have to be a little more 'high-techy', or 'techno-geek' to really appreciate the digitals.
Last word, IMHO, for $399, feature-for-feature, the Tascam 424mkIII cassette 4-track Portastudio is better value than the Boss BR-532 and Korg PXR4 digital tinker-toys.
Good luck, & keep us posted on what you buy. Inquiring minds want to know!