As far as the main question goes, I'm mostly posting to follow. I'd be glad to find out what the best cell phone option for international travel is.
When I've travelled internationally in the past couple years, I've generally purchased a temporary SIM card for the country and then used email, Facebook Messenger, WhatsApp, and Skype to deal with people back in the US or UK. When I travelled around 2017 to 2019, I didn't usually bother with the international SIM card and just used the apps via WIFI at my hotel or at coffee shops or train stations. Regardless of what option you go with for the phone service itself, I think it would be a good idea for the OP and son to both be on the same app to communicate via WIFI or data, just in case cell phone service doesn't work out as anticipated.
WhatsApp works well, and as FIRE 20/20 said, many people in Europe use it already. Your son might want it to keep up with people met on the trip. Facebook Messenger is another free option. Both of those will let you share messages and photos or make voice or video calls. WhatsApp feels a little easier because you can reach people from your existing phone contacts, and group chats work a little more smoothly. I came back to the US from the UK almost two years ago, and I still feel surprised when people want to make group chats via actual cell text messages instead of WhatsApp. It feels a bit clunky now that I'm used to group chats on WhatsApp, but I guess cell phone text messages still work reasonably well in North America because most people you meet all have US or Canada phone numbers. Young people seem to like Instagram for keeping in touch, but the messaging feels like an afterthought rather than the main function.