I've had a phone since 3rd grade I think? I walked a few miles home everyday, so it was just a way to keep in touch for those just-in-case times. I of course always broke my phone and I never really used the phone, so I always had the cheapest plastic phone. It depends on how your son treats other things he has; if he isn't cautious with everything, then it may be best of stick with a more basic phone. I don't really see the need for an iPhone. I see tons of teens with iPhones these days; I can't imagine what 99% of them need all that functionality for. Making a call was enough
maybe Snake. I'm just speaking from experience as a former child (well weren't we all).
I believe that the cell phone for any child should be nothing more than a communication tool. It should work in most conditions. I'd disable SMS, MMS, and Internet. Obviously with an iPhone you cannot block the Internet. Do you really need you son browsing the Internet in school when he should be doing work?
Of course if your son is in college, or something then all this is moot. Go for it.
Don't forget any limitation that AT&T offers comes at an additional cost; you're paying like $10 a month for Smart Limits to limit spending. Same thing with adding a data plan to curb data spending, a messaging plan to limit messaging spending.