WiFi is just a connection between a mobile device and broadband router, this has nothing to do with roaming.
If your son has taken a pay-monthly phone abroad without activating roaming, this will prevent connections to foreign networks and result in a lack of coverage. That causes an inability to use functions that rely on network service - calls, texts & mobile data.
The account holder will need to contact EE CS and request roaming is enabled. If your son is the account holder, he will need to call using an alternative means - perhaps using a web-calling app such as Skype over a WiFi connection. If you are the account holder, you may do so by calling 150 from an EE phone, or 0800 0798586 from any other phone.
