Can motion controls be changed???

Without really understanding your sons needs, I would say that the iPad is very responsive to a variety motion inputs, either fast or slow. There are many reports of very young children using the iPad intuitively and without much problem. The best thing would be for him to try it.

But to answer your question, you cannot change the sensitivity in a non-Jailbroken iPad. In the JB world, there are a number of programs that allow more control over settings, but I couldn't say if motion sensitivity is one.

 

I'm not entirely clear what you mean by motion controls. You can lock the orientation of an iPad so that moving it doesn't rotate the screen. On the current software this can be done by flipping the physical switch; on the upcoming 4.2 software you'll need to double tap the home button and swipe to the right to find the orientation lock icon.

Some games do use motion controls to, for example, turn the iPad into a steering wheel. Normally these games seem to have a built in sensitivity setting.

Good luck with trying out an iPad

 

iPad doesn't have a mouse pointer, you tap the icons and other on-screen controls directly with your fingers.

NY Times recently ran an article about how an iPad was being used for a boy with disabilities, perhaps it might have useful information for you.
http://www.nytimes.com/2010/10/31/nyregion/31owen.html?_r=4&scp=2&sq=ipad&st=cse

Good luck with finding the right assistive equipement for your son.