I need some advice on Raspberry Pi for my son who is interested in Robotics.

Hi there Keith, it's always hard to recommend a particular course of action with out knowing the capabilities and prior knowledge of your son.

Assuming he currently knows little but is willing to learn I would say that the first task is to learn a programming language any will do but Python is particularly well supported on the 'PI.

Once you are somewhat familiar with the basics I would then take the step of trying to interface some simple sensors(or at least switches and LEDs)  to the Pi.

https://projects.drogon.net/raspberry-pi/gpio-examples/tux-crossing/gpio-examples-1-a-single-led/

How you do this doesn't matter there are many online tutorials .What ever suits your son is best , once you have learned simple I/O you can get a feel how the more complex robot systems work.

I belive shabaz has plans for a nice system so keep an eye out over the next few months

There was recently a RoadTest about the Rapiro robot based on the Raspberry Pi. It costs about $450 though, so it might be outside of your budget.

It looks cute but really there's not much to learn for that $450

I've used RasPi a lot, but not for robotics, so YMMV.  But I would suggest starting with some of the RasPi motor control projects at Adafruit: https://learn.adafruit.com/category/raspberry-pi

I suspect the parts costs for these projects are pretty low, especially if you salvage motors from dead printers.

Then if he's still interested, give him a budget and let him select his own parts.

Really? The reviews for it were really good. I thought it would be better than that.

not really educational material or price point though I think..Still a very cool design

The kits ment for Arduinos often have cheap motors and stuff and are an accessible way of getting parts.

Scraping printers etc as John Says is an excellent  way of learning and getting some very nice parts ..sometimes they are abit hard to identify however but most of the time you are all right!

I think arduino based robots are significantly simpler (easier to learn) and less costly than Raspberry Pi robots.

However, if you like Raspberry Pi there is a lot of power there. The Raspberry Pi needs an interface card to connect motors and sensors and there are several choices, such as PiFace Digital or GertBoard.

One solution I like is a ChipKit Pi which is a powerful peripheral controller that allows inexpensive arduino peripherals to be used with the Raspberry Pi and all softtware development can be performed on the Raspberry Pi using their free development environment.

The Little British Robot Company have just launched a very low cost Raspberry Pi Robot on kickstarter

They reached their target. Now they have to provide the rewards to their backers, but they will soon be putting them in their shop,

If your son can wait that would be ideal. If not, SK Pang offer a driver kit, but your son will need to get motors and a base.

If he wants to go that route I can give more detailed advice.

I Agree for most simple applications the bots based on Arduinos are probably more appropriate, how ever in both cases they need extra motor driver boards or whatever to drive much more than an LED.