If you suspend service on the phone, it means that the phone will not be able to access the cellular network. However, if the phone is connected to WiFi, it will still be able to access the Internet. It won't be able to make/receive calls, or send/receive text messages.
If your son has an iphone he will still be able to messages to other apple users over wifi using iMessage. Also there are lots of wifi alternative texting apps for android and iOS. BB10 devices can use bbm over wifi so texting is possible.