Who is your carrier and what kind of phone does your son have? I am not familiar with the idea of an integrated SIM. You may be surprised at which US phones will indeed work in Europe.
My high-school age son went to Copenhagen last fall on a student exchange program. I agree with the recommendations above for connecting to WiFi and using Skype/Facebook Messenger/What’s App or other options. I have limited experience with Google Voice but that is probably also a great option. There is no cost at all for any of these. They can be used for instant messaging, voice, or video calls. WiFi is so ubiquitous now that this is the most sensible option. Depending on his phone he probably doesn’t need to do anything at all but drop his phone in his pocket.
I have used three other methods for phone use abroad. One is getting a local SIM card when I arrive and plugging that into my phone. I have Verizon and have used an LG LG3, Samsung Galaxy 3, HTC Thunderbolt, and NV Touch, and IIRC all have supported GSM 900.
Another is to set up a temporary international calling plan with your carrier. I have done this with Verizon, where you can get on an international plan for one billing period (or you can get it prorated if your trip crosses the billing period but it’s an administrative pain). This includes some combination of voice and data. But if you have WiFi the first option above is better.
I have a Mobal account. I signed up several years ago when they were a good option but quit a long time ago because their rates are expensive compared to the other options listed above. You do get a dedicated phone number but it’s a UK phone number. Fine if you’re from the UK but if you’re a Yank friends in the U.S. who call you have to pay for an international call. I also tried another similar service whose name escapes me at the moment. I canceled because their web site says that the number includes voicemail but it turned it that was false. I needed it to stay in touch at work and the lack of voicemail was a non-starter.