I just carried a backpack as my “personal item” on the plane and it worked very well. I could see sending DS16 with a carry on suitcase and a backpack. He would have trouble with a large checked bag. The itinerary is Rome, Florence, Capri, Sorrento Region, Pompeii. I think there are 5 or 6 days in Rome. The teacher opted for fewer stops so that they could spend more time in each place, which we thought was wise.
On 3 trips to London from 2014 - 17 I got the Verizon international plan, but I did call home and also used all the features of my phone. That being said, these things may have changed since then–a lot. I am not tech savvy and will have to show the suggestions to DH and DS14, who understands phones and computers far better than I do.
I’m not gonna comment from the parent end but I went to Italy for 10 days when I was 15 with a school group too. We had no disasters other than I spent the entire money my mom gave me for souvenirs on a blown glass for her early on in Venice (which she still has) and had to skip some lunches at the end (breakfasts and dinners were always covered). Life lessons! ;-). I think my mom gave me an emergency credit card too but I didn’t want to use it.
Just to be clear on the phone. It will work whether you get the international plan or not. If he’s lost on the side of the road he can make a call or text or even turn on data and pull up a map. This will not be cheap but it’s available in an emergency. If he turns off mobile data and do not make phone calls or texts via the phone service is what I would recommend. There is enough free wifi now that he can make calls or send messages via WhatsApp or any other message service for free. If you can’t trust him not to call/text off of wifi then I would investigate the International Plan.
Then he’ll be just fine looking left, right, left crossing the street!
Send him with a couple of travel-sized bottles of woolite. When you arrive at lodging, it is easy/quick to wash the previous day’s dirties in the sink and lay on a towel on the bed. By morning they are dry.
The main thing will be avoiding pickpockets. Zippered pockets or those waist money belts don’t even slow them down. I have a black leather necklace safe that I prefer for the passport. It is on a black leather thong, so could just be a funky necklace. If you see kids running amok, start swinging your arms around you wildly. Don’t worry if you wack one, if they get close enough to be wacked they were trying to steal from you. He’ll be pegged for what he is, but when he isn’t with the big group, stay silent when in doubt. I also usually avoided the traditional American dress, but as a teen boy he probably has nothing else.