We did this for our kid who went to Italy and Australia recently. My husband went online at T-Mobile and paid $50 for their international data pass that gave her 30 days of 15G and unlimited calling (and face-timing). We set the start date based on her flight schedule. Didn’t need the 30 days but the next lower option was 10 days and that wasn’t enough. There are data limits, so you have to watch how long you talk and don’t go over the limit. Same SIM card, same phone, didn’t have to change anything there.
I hope your son has fun!