I think the most important thing is to keep it simple. EXTREMELY simple. Set it up for him, moving ALL the apps out of the home screen except the phone, iMessage, and MAYBE email or camera or two or three at the most. Let him get used to a few things first, and then one by one give him an app that does what he's missing. Too much at once gets overwhelming, but if it's extremely simple, they get used to things quick because they are able to use them, and can more easily handle new features (apps) as you add them over time.
Don't even BEGIN to try to explain iCloud and syncing and stuff like that, at least not at first. Phone, iMessages, Camera, and FB Messenger and they will be THRILLED. After a week or so then add a little at a time.
Remember, features that would impress you are more likely to overwhelm them.