That leaves AT&T as your ONLY option if you need voice and 3G data. Here's a link to their GoPhone prepaid plans webpage. As you'll see, prepaid data (required for a smartphone) is pretty steep ($15 for 100 MB, $25 for 500 MB) though probably cheaper than what you'd pay using your home carrier overseas.
If you think you'll have access to a lot of Wifi, T-Mobile is best, otherwise you're best/only real choice is AT&T.
This is the only part of your letter that has any relevance to him using his iPhone over here. As far as useful info, many have posted, ATT or tmobile. That's it. Why go further??
OP, Trust me, stick with ATT. You're not going be stuck at mcdonalds or the hotel while you're on vacation. So saying you're going to be around wifi all the time may be true but your not going be connected to it.
For as much as it cost to take a vacation like this, I think another 50-100 is worth it to do exactly what you want. Tweet, FB, maps, whatever.