The SIM card slot is on the top, next to the earphone jack. Look in his iPhone box for the "SIM Eject tool," or just straighten out a small paper clip to eject the SIM tray.
I'm not sure about this, but you might need to "unbrick" the iPhone once you put that Blackberry SIM in there. That means hooking it up to iTunes, and probably changing the account over to an iPhone data plan. Is your Uncle switching over for good?
Once that's done, go to Settings, then to "Mail, Contacts, Calendars." Scroll down and you should see a button for "Import SIM contacts." That's what you need.