The M-Audio would be my choice, though you ought to verify that whatever he has now isn't already 5.1 capable.
If you wish to use regular speakers, you'll also need a cheap receiver.
Klipsch and Logitech both make well received speaker packages as do Cambridge Soundwerks. The logitechs are usually the cheapest but still do pretty well. The Z640s (their wimpier version) are available at Gateway.com:
"Search for item 6612502 to find the Logitech z640 5.1 Speaker Set priced at $54 shipped using free shipping code NOSHIPA."
-From BensBargains.net
As to placement, would it be possible to mount them to his chair?
