Unfortuntely you'll probably have to buy Powerpoint. Powerpoint is considered an 'Office ie. corporate' product so it doesn't come standard with most home versions.
If you have a Windows machine, you can usually view a ppt document but not create or edit (a bit like Adobe PDF).
For a cheap and legal option check out 'It's not cheating' if your son is in school or uni.http://www.microsoft.com/student/discounts...ting/terms.aspx
I have to admit it's a bit disappointing that the school expects parents to have Powerpoint. Alternatively, you could create a presentation (but no animation functionality) using Word and then PDF it.
If you have an iPad or Mac you can use Keynote. I *think* it's $22 from the app store. Certainly from a business perspective, a lot of people are now using Keynote as they can plug their iPad directly into a tv and not lug a laptop around.