Personally, I think it is an AT&T problem and not an iPhone problem. Why?
History: We had AT&T and were getting these charges, my daughter's iPhone especially. Hourly charges were occurring when she was out of house or the phone went to sleep. My other daughter's Samsung got them when she was out of the house, away from the WiFi. The iPhone would not REconnect to the WiFi on its own. My iPhone had no problem. So after visiting Apple about six times, they fixed this problem. She still had hourly charges occurring, though. I shut the data off on my phone, 99% of the time it sits ontop of the WiFi router.
I then got POed at AT&T's customer service, so we switched to Verizon. We got two iPhones (4S and 4) and one Samsung. I told both kids to use the phones as much as they wanted or was usual, put all their apps on,let my 5 yo granddaughter play. I flipped my data back on and watched the bill.
Guess what? When we go to sleep so do the phones! Looking at her data use from the 11th, she had two entries, at 7:04 a.m. and 7:04 p.m., approximate total of 1,000 kb. For the 12th, first at 7:04 a.m. until 8:33 p.m., 13th first was 8:31 a.m., last 3:02 p.m. (only 2). For the 14th, again only two, 2:02 a.m. and 1:02 p.m. The 15th was two also, 6:04 a.m. and 11:24 p.m. The 16th were a bunch (she was texting a lot), the first at 6:05 a.m. and last 10:07 p.m. and the 17th was 9:49 a.m. and 6:49 p.m. Oddly enough, there were none for the 18th. Now, she works outside of the house, so her phone is basically alseep most of the weekdays, as well as at night when she sleeps. She wakes around 5:30 on weekdays.
You can draw your own conclusions, but I seriously do not think this is an iPhone thing, but an AT&T thing. Due to the fact that we could not keep our AT&T phones when we switched (which is normal), it looks like to me that AT&T has programmed the phones for these pushes for whatever reason and for whatever reason, they can't be done through WiFi on iPhones. Our two iPhones are now sleeping just fine now that they are on Verizon and our data use is way down. The Samsung was never a probem. When it's in the house, it's connected to WiFi. My phone, the iPhone 4 is sending wee little bits of information that I don't particularly understand since I had one app (flashlight), I rarely leave the house since I work at home and it just sits around asleep most of the time. My average daily use for the data is 100 kb, so it's not real concerning.
BTW, Mark, depending on how much your son uses 'data', I shut mine off on the phone when we were with AT&T. It is easy enough to turn back on when needed. It would, however, be a pain in 'you know where' if the data was being used a lot during the day. It's a thought, might not be a 'good' thought.
PS: Neither of us have or did have email set up through the phones. We needed the necessary gmail accounts or whatever, but mine is not active nor is hers, or ever was.